diff --git a/drizzle.test.config.ts b/drizzle.test.config.ts
new file mode 100644
index 0000000..9f6dcc5
--- /dev/null
+++ b/drizzle.test.config.ts
@@ -0,0 +1,11 @@
+import { defineConfig } from 'drizzle-kit';
+
+if (!process.env.TEST_DATABASE_URL) throw new Error('TEST_DATABASE_URL is not set');
+
+export default defineConfig({
+	schema: './src/lib/server/db/schema.ts',
+	dialect: 'sqlite',
+	dbCredentials: { url: process.env.TEST_DATABASE_URL },
+	verbose: true,
+	strict: true
+});
\ No newline at end of file
diff --git a/src/lib/server/auth.test.ts b/src/lib/server/auth.test.ts
new file mode 100644
index 0000000..a7e835f
--- /dev/null
+++ b/src/lib/server/auth.test.ts
@@ -0,0 +1,115 @@
+import type { RequestEvent } from '@sveltejs/kit';
+import { eq } from 'drizzle-orm';
+import { sha256 } from '@oslojs/crypto/sha2';
+import { encodeBase64url, encodeHexLowerCase } from '@oslojs/encoding';
+import { testDb as db } from '$lib/server/db/test';
+import * as table from '$lib/server/db/schema';
+
+const DAY_IN_MS = 1000 * 60 * 60 * 24;
+
+export const sessionCookieName = 'auth-session';
+
+export function generateSessionToken() {
+	const bytes = crypto.getRandomValues(new Uint8Array(18));
+	const token = encodeBase64url(bytes);
+	return token;
+}
+
+export async function createSession(token: string, userId: string) {
+	const sessionId = encodeHexLowerCase(sha256(new TextEncoder().encode(token)));
+	const session: table.Session = {
+		id: sessionId,
+		userId,
+		expiresAt: new Date(Date.now() + DAY_IN_MS * 30)
+	};
+	await db.insert(table.session).values(session);
+	return session;
+}
+
+export async function validateSessionToken(token: string) {
+	const sessionId = encodeHexLowerCase(sha256(new TextEncoder().encode(token)));
+	const [result] = await db
+		.select({
+			// Adjust user table here to tweak returned data
+			user: { id: table.user.id, email: table.user.email },
+			session: table.session
+		})
+		.from(table.session)
+		.innerJoin(table.user, eq(table.session.userId, table.user.id))
+		.where(eq(table.session.id, sessionId));
+
+	if (!result) {
+		return { session: null, user: null };
+	}
+	const { session, user } = result;
+
+	const sessionExpired = Date.now() >= session.expiresAt.getTime();
+	if (sessionExpired) {
+		await db.delete(table.session).where(eq(table.session.id, session.id));
+		return { session: null, user: null };
+	}
+
+	const renewSession = Date.now() >= session.expiresAt.getTime() - DAY_IN_MS * 15;
+	if (renewSession) {
+		session.expiresAt = new Date(Date.now() + DAY_IN_MS * 30);
+		await db
+			.update(table.session)
+			.set({ expiresAt: session.expiresAt })
+			.where(eq(table.session.id, session.id));
+	}
+
+	return { session, user };
+}
+
+export type SessionValidationResult = Awaited<ReturnType<typeof validateSessionToken>>;
+
+export async function invalidateSession(sessionId: string) {
+	await db.delete(table.session).where(eq(table.session.id, sessionId));
+}
+
+export function setSessionTokenCookie(event: RequestEvent, token: string, expiresAt: Date) {
+	event.cookies.set(sessionCookieName, token, {
+		expires: expiresAt,
+		path: '/'
+	});
+}
+
+export function deleteSessionTokenCookie(event: RequestEvent) {
+	event.cookies.delete(sessionCookieName, {
+		path: '/'
+	});
+}
+
+export async function hashPassword(password: string): Promise<string> {
+	return await Bun.password.hash(password, {
+		algorithm: 'argon2id',
+		memoryCost: 4,
+		timeCost: 3
+	});
+}
+
+export async function verifyPassword(password: string, hash: string): Promise<boolean> {
+	try {
+		return await Bun.password.verify(password, hash);
+	} catch {
+		return false;
+	}
+}
+
+export async function createUser(anonymousId: string, email: string, passwordHash: string, firstName?: string, lastName?: string) {
+	const user: table.User = {
+		id: anonymousId, // Use anonymousId as the user ID to preserve stats
+		email,
+		passwordHash,
+		firstName: firstName || null,
+		lastName: lastName || null,
+		isPrivate: false
+	};
+	await db.insert(table.user).values(user);
+	return user;
+}
+
+export async function getUserByEmail(email: string) {
+	const [user] = await db.select().from(table.user).where(eq(table.user.email, email));
+	return user || null;
+}
\ No newline at end of file
diff --git a/src/lib/server/db/test.ts b/src/lib/server/db/test.ts
new file mode 100644
index 0000000..1adee5c
--- /dev/null
+++ b/src/lib/server/db/test.ts
@@ -0,0 +1,9 @@
+import { drizzle } from 'drizzle-orm/bun-sqlite';
+import { Database } from 'bun:sqlite';
+import * as schema from './schema';
+
+if (!Bun.env.TEST_DATABASE_URL) throw new Error('TEST_DATABASE_URL is not set');
+
+const testClient = new Database(Bun.env.TEST_DATABASE_URL);
+
+export const testDb = drizzle(testClient, { schema });
\ No newline at end of file
diff --git a/tests/signin-migration-unit.test.ts b/tests/signin-migration-unit.test.ts
new file mode 100644
index 0000000..b6427c4
--- /dev/null
+++ b/tests/signin-migration-unit.test.ts
@@ -0,0 +1,245 @@
+import { describe, it, expect, beforeEach, afterEach } from "bun:test";
+
+describe('Signin Migration Logic (Unit Tests)', () => {
+	
+	// Test the deduplication algorithm independently
+	it('should correctly identify and remove duplicates keeping earliest', () => {
+		// Mock completion data structure
+		type MockCompletion = {
+			id: string;
+			anonymousId: string;
+			date: string;
+			guessCount: number;
+			completedAt: Date;
+		};
+
+		// Test data: multiple completions on same date
+		const allUserCompletions: MockCompletion[] = [
+			{
+				id: 'comp1',
+				anonymousId: 'user123',
+				date: '2024-01-01',
+				guessCount: 4,
+				completedAt: new Date('2024-01-01T08:00:00Z') // Earliest
+			},
+			{
+				id: 'comp2', 
+				anonymousId: 'user123',
+				date: '2024-01-01',
+				guessCount: 2,
+				completedAt: new Date('2024-01-01T14:00:00Z') // Later
+			},
+			{
+				id: 'comp3',
+				anonymousId: 'user123',
+				date: '2024-01-01',
+				guessCount: 6,
+				completedAt: new Date('2024-01-01T20:00:00Z') // Latest
+			},
+			{
+				id: 'comp4',
+				anonymousId: 'user123',
+				date: '2024-01-02',
+				guessCount: 3,
+				completedAt: new Date('2024-01-02T09:00:00Z') // Unique date
+			}
+		];
+
+		// Implement the deduplication logic from signin server action
+		const dateGroups = new Map<string, MockCompletion[]>();
+		for (const completion of allUserCompletions) {
+			const date = completion.date;
+			if (!dateGroups.has(date)) {
+				dateGroups.set(date, []);
+			}
+			dateGroups.get(date)!.push(completion);
+		}
+
+		// Process dates with duplicates
+		const duplicateIds: string[] = [];
+		const keptEntries: MockCompletion[] = [];
+		
+		for (const [date, completions] of dateGroups) {
+			if (completions.length > 1) {
+				// Sort by completedAt timestamp (earliest first)
+				completions.sort((a, b) => a.completedAt.getTime() - b.completedAt.getTime());
+				
+				// Keep the first (earliest), mark the rest for deletion
+				const toKeep = completions[0];
+				const toDelete = completions.slice(1);
+				
+				keptEntries.push(toKeep);
+				duplicateIds.push(...toDelete.map(c => c.id));
+			} else {
+				// Single entry for this date, keep it
+				keptEntries.push(completions[0]);
+			}
+		}
+
+		// Verify the logic worked correctly
+		expect(duplicateIds).toHaveLength(2); // comp2 and comp3 should be deleted
+		expect(duplicateIds).toContain('comp2');
+		expect(duplicateIds).toContain('comp3');
+		expect(duplicateIds).not.toContain('comp1'); // comp1 should be kept (earliest)
+		expect(duplicateIds).not.toContain('comp4'); // comp4 should be kept (unique date)
+
+		// Verify kept entries
+		expect(keptEntries).toHaveLength(2);
+		
+		// Check that the earliest entry for 2024-01-01 was kept
+		const jan1Entry = keptEntries.find(e => e.date === '2024-01-01');
+		expect(jan1Entry).toBeTruthy();
+		expect(jan1Entry!.id).toBe('comp1'); // Earliest timestamp
+		expect(jan1Entry!.guessCount).toBe(4);
+		expect(jan1Entry!.completedAt.getTime()).toBe(new Date('2024-01-01T08:00:00Z').getTime());
+
+		// Check that unique date entry was preserved
+		const jan2Entry = keptEntries.find(e => e.date === '2024-01-02');
+		expect(jan2Entry).toBeTruthy();
+		expect(jan2Entry!.id).toBe('comp4');
+	});
+
+	it('should handle no duplicates correctly', () => {
+		type MockCompletion = {
+			id: string;
+			anonymousId: string;
+			date: string;
+			guessCount: number;
+			completedAt: Date;
+		};
+
+		// Test data: all unique dates
+		const allUserCompletions: MockCompletion[] = [
+			{
+				id: 'comp1',
+				anonymousId: 'user123',
+				date: '2024-01-01',
+				guessCount: 4,
+				completedAt: new Date('2024-01-01T08:00:00Z')
+			},
+			{
+				id: 'comp2',
+				anonymousId: 'user123', 
+				date: '2024-01-02',
+				guessCount: 2,
+				completedAt: new Date('2024-01-02T14:00:00Z')
+			}
+		];
+
+		// Run deduplication logic
+		const dateGroups = new Map<string, MockCompletion[]>();
+		for (const completion of allUserCompletions) {
+			if (!dateGroups.has(completion.date)) {
+				dateGroups.set(completion.date, []);
+			}
+			dateGroups.get(completion.date)!.push(completion);
+		}
+
+		const duplicateIds: string[] = [];
+		for (const [date, completions] of dateGroups) {
+			if (completions.length > 1) {
+				completions.sort((a, b) => a.completedAt.getTime() - b.completedAt.getTime());
+				const toDelete = completions.slice(1);
+				duplicateIds.push(...toDelete.map(c => c.id));
+			}
+		}
+
+		// Should find no duplicates
+		expect(duplicateIds).toHaveLength(0);
+	});
+
+	it('should handle edge case with same timestamp', () => {
+		type MockCompletion = {
+			id: string;
+			anonymousId: string;
+			date: string;
+			guessCount: number;
+			completedAt: Date;
+		};
+
+		// Edge case: same completion time (very unlikely but possible)
+		const sameTime = new Date('2024-01-01T08:00:00Z');
+		const allUserCompletions: MockCompletion[] = [
+			{
+				id: 'comp1',
+				anonymousId: 'user123',
+				date: '2024-01-01',
+				guessCount: 3,
+				completedAt: sameTime
+			},
+			{
+				id: 'comp2',
+				anonymousId: 'user123',
+				date: '2024-01-01', 
+				guessCount: 5,
+				completedAt: sameTime
+			}
+		];
+
+		// Run deduplication logic
+		const dateGroups = new Map<string, MockCompletion[]>();
+		for (const completion of allUserCompletions) {
+			if (!dateGroups.has(completion.date)) {
+				dateGroups.set(completion.date, []);
+			}
+			dateGroups.get(completion.date)!.push(completion);
+		}
+
+		const duplicateIds: string[] = [];
+		for (const [date, completions] of dateGroups) {
+			if (completions.length > 1) {
+				completions.sort((a, b) => a.completedAt.getTime() - b.completedAt.getTime());
+				const toDelete = completions.slice(1);
+				duplicateIds.push(...toDelete.map(c => c.id));
+			}
+		}
+
+		// Should still remove one duplicate (deterministically based on array order)
+		expect(duplicateIds).toHaveLength(1);
+		// Since they have the same timestamp, it keeps the first one in the sorted array
+		expect(duplicateIds[0]).toBe('comp2'); // Second entry gets removed
+	});
+
+	it('should validate migration condition logic', () => {
+		// Test the condition check that determines when migration should occur
+		const testCases = [
+			{ 
+				anonymousId: 'device2-id',
+				userId: 'device1-id',
+				shouldMigrate: true,
+				description: 'Different IDs should trigger migration'
+			},
+			{
+				anonymousId: 'same-id',
+				userId: 'same-id', 
+				shouldMigrate: false,
+				description: 'Same IDs should not trigger migration'
+			},
+			{
+				anonymousId: null as any,
+				userId: 'user-id',
+				shouldMigrate: false,
+				description: 'Null anonymous ID should not trigger migration'
+			},
+			{
+				anonymousId: undefined as any,
+				userId: 'user-id',
+				shouldMigrate: false,
+				description: 'Undefined anonymous ID should not trigger migration'
+			},
+			{
+				anonymousId: '',
+				userId: 'user-id', 
+				shouldMigrate: false,
+				description: 'Empty anonymous ID should not trigger migration'
+			}
+		];
+
+		for (const testCase of testCases) {
+			// This is the exact condition from signin/+page.server.ts
+			const shouldMigrate = !!(testCase.anonymousId && testCase.anonymousId !== testCase.userId);
+			
+			expect(shouldMigrate).toBe(testCase.shouldMigrate);
+		}
+	});
+});
\ No newline at end of file
diff --git a/tests/signin-migration.test.ts b/tests/signin-migration.test.ts
new file mode 100644
index 0000000..63c1c54
--- /dev/null
+++ b/tests/signin-migration.test.ts
@@ -0,0 +1,287 @@
+import { describe, it, expect, beforeEach, afterEach } from "bun:test";
+import { testDb as db } from '../src/lib/server/db/test';
+import { user, session, dailyCompletions } from '../src/lib/server/db/schema';
+import * as auth from '../src/lib/server/auth.test';
+import { eq, inArray } from 'drizzle-orm';
+import crypto from 'node:crypto';
+
+// Test helper functions
+function generateTestUUID() {
+	return crypto.randomUUID();
+}
+
+async function createTestUser(anonymousId: string, email: string, password: string = 'testpass123') {
+	const passwordHash = await auth.hashPassword(password);
+	const testUser = await auth.createUser(anonymousId, email, passwordHash, 'Test', 'User');
+	return testUser;
+}
+
+async function createTestCompletion(anonymousId: string, date: string, guessCount: number, completedAt: Date) {
+	const completion = {
+		id: generateTestUUID(),
+		anonymousId,
+		date,
+		guessCount,
+		completedAt
+	};
+	await db.insert(dailyCompletions).values(completion);
+	return completion;
+}
+
+async function clearTestData() {
+	// Clear test data in reverse dependency order
+	await db.delete(session);
+	await db.delete(dailyCompletions);
+	await db.delete(user);
+}
+
+describe('Signin Stats Migration', () => {
+	beforeEach(async () => {
+		await clearTestData();
+	});
+
+	afterEach(async () => {
+		await clearTestData();
+	});
+
+	it('should migrate stats from local anonymous ID to user ID on signin', async () => {
+		// Setup: Create user with device 1 anonymous ID
+		const device1AnonymousId = generateTestUUID();
+		const device2AnonymousId = generateTestUUID();
+		const email = 'test@example.com';
+		
+		const testUser = await createTestUser(device1AnonymousId, email);
+		
+		// Add some completions for device 1 (user's original device)
+		await createTestCompletion(device1AnonymousId, '2024-01-01', 3, new Date('2024-01-01T08:00:00Z'));
+		await createTestCompletion(device1AnonymousId, '2024-01-02', 5, new Date('2024-01-02T09:00:00Z'));
+		
+		// Add some completions for device 2 (before signin)
+		await createTestCompletion(device2AnonymousId, '2024-01-03', 2, new Date('2024-01-03T10:00:00Z'));
+		await createTestCompletion(device2AnonymousId, '2024-01-04', 4, new Date('2024-01-04T11:00:00Z'));
+		
+		// Verify initial state
+		const initialDevice1Stats = await db
+			.select()
+			.from(dailyCompletions)
+			.where(eq(dailyCompletions.anonymousId, device1AnonymousId));
+		const initialDevice2Stats = await db
+			.select()
+			.from(dailyCompletions)
+			.where(eq(dailyCompletions.anonymousId, device2AnonymousId));
+			
+		expect(initialDevice1Stats).toHaveLength(2);
+		expect(initialDevice2Stats).toHaveLength(2);
+		
+		// Simulate signin action - this is what happens in signin/+page.server.ts
+		const user = await auth.getUserByEmail(email);
+		expect(user).toBeTruthy();
+		
+		// Migrate stats (simulating the signin logic)
+		if (device2AnonymousId && device2AnonymousId !== user!.id) {
+			// Update all daily completions from device2 anonymous ID to user's ID
+			await db
+				.update(dailyCompletions)
+				.set({ anonymousId: user!.id })
+				.where(eq(dailyCompletions.anonymousId, device2AnonymousId));
+		}
+		
+		// Verify migration worked
+		const finalUserStats = await db
+			.select()
+			.from(dailyCompletions)
+			.where(eq(dailyCompletions.anonymousId, user!.id));
+		const remainingDevice2Stats = await db
+			.select()
+			.from(dailyCompletions)
+			.where(eq(dailyCompletions.anonymousId, device2AnonymousId));
+			
+		expect(finalUserStats).toHaveLength(4); // All 4 completions now under user ID
+		expect(remainingDevice2Stats).toHaveLength(0); // No more completions under device2 ID
+		
+		// Verify the actual data is correct
+		const dates = finalUserStats.map(c => c.date).sort();
+		expect(dates).toEqual(['2024-01-01', '2024-01-02', '2024-01-03', '2024-01-04']);
+	});
+
+	it('should deduplicate entries for same date keeping earliest completion', async () => {
+		// Setup: User played same day on both devices
+		const device1AnonymousId = generateTestUUID();
+		const device2AnonymousId = generateTestUUID();
+		const email = 'test@example.com';
+		
+		const testUser = await createTestUser(device1AnonymousId, email);
+		
+		// Both devices played on same date - device1 played earlier and better
+		const date = '2024-01-01';
+		const earlierTime = new Date('2024-01-01T08:00:00Z');
+		const laterTime = new Date('2024-01-01T14:00:00Z');
+		
+		await createTestCompletion(device1AnonymousId, date, 3, earlierTime); // Better score, earlier
+		await createTestCompletion(device2AnonymousId, date, 5, laterTime);   // Worse score, later
+		
+		// Also add unique dates to ensure they're preserved
+		await createTestCompletion(device1AnonymousId, '2024-01-02', 4, new Date('2024-01-02T09:00:00Z'));
+		await createTestCompletion(device2AnonymousId, '2024-01-03', 2, new Date('2024-01-03T10:00:00Z'));
+		
+		// Migrate stats
+		const user = await auth.getUserByEmail(email);
+		await db
+			.update(dailyCompletions)
+			.set({ anonymousId: user!.id })
+			.where(eq(dailyCompletions.anonymousId, device2AnonymousId));
+		
+		// Implement deduplication logic (from signin server action)
+		const allUserCompletions = await db
+			.select()
+			.from(dailyCompletions)
+			.where(eq(dailyCompletions.anonymousId, user!.id));
+
+		// Group by date to find duplicates
+		const dateGroups = new Map<string, typeof allUserCompletions>();
+		for (const completion of allUserCompletions) {
+			const date = completion.date;
+			if (!dateGroups.has(date)) {
+				dateGroups.set(date, []);
+			}
+			dateGroups.get(date)!.push(completion);
+		}
+
+		// Process dates with duplicates
+		const duplicateIds: string[] = [];
+		for (const [date, completions] of dateGroups) {
+			if (completions.length > 1) {
+				// Sort by completedAt timestamp (earliest first)
+				completions.sort((a, b) => a.completedAt.getTime() - b.completedAt.getTime());
+				
+				// Keep the first (earliest), mark the rest for deletion
+				const toDelete = completions.slice(1);
+				duplicateIds.push(...toDelete.map(c => c.id));
+			}
+		}
+
+		// Delete duplicate entries
+		if (duplicateIds.length > 0) {
+			await db
+				.delete(dailyCompletions)
+				.where(inArray(dailyCompletions.id, duplicateIds));
+		}
+		
+		// Verify deduplication worked correctly
+		const finalStats = await db
+			.select()
+			.from(dailyCompletions)
+			.where(eq(dailyCompletions.anonymousId, user!.id));
+			
+		expect(finalStats).toHaveLength(3); // One duplicate removed
+		
+		// Verify the correct entry was kept for the duplicate date
+		const duplicateDateEntry = finalStats.find(c => c.date === date);
+		expect(duplicateDateEntry).toBeTruthy();
+		expect(duplicateDateEntry!.guessCount).toBe(3); // Better score kept
+		expect(duplicateDateEntry!.completedAt.getTime()).toBe(earlierTime.getTime()); // Earlier time kept
+		
+		// Verify unique dates are preserved
+		const allDates = finalStats.map(c => c.date).sort();
+		expect(allDates).toEqual(['2024-01-01', '2024-01-02', '2024-01-03']);
+	});
+
+	it('should handle no migration when anonymous ID matches user ID', async () => {
+		// Setup: User signing in from same device they signed up on
+		const anonymousId = generateTestUUID();
+		const email = 'test@example.com';
+		
+		const testUser = await createTestUser(anonymousId, email);
+		
+		// Add some completions
+		await createTestCompletion(anonymousId, '2024-01-01', 3, new Date('2024-01-01T08:00:00Z'));
+		await createTestCompletion(anonymousId, '2024-01-02', 5, new Date('2024-01-02T09:00:00Z'));
+		
+		// Verify initial state
+		const initialStats = await db
+			.select()
+			.from(dailyCompletions)
+			.where(eq(dailyCompletions.anonymousId, anonymousId));
+		expect(initialStats).toHaveLength(2);
+		
+		// Simulate signin with same anonymous ID (no migration needed)
+		const user = await auth.getUserByEmail(email);
+		
+		// Migration logic should skip when IDs match
+		const shouldMigrate = anonymousId && anonymousId !== user!.id;
+		expect(shouldMigrate).toBe(false);
+		
+		// Verify no changes
+		const finalStats = await db
+			.select()
+			.from(dailyCompletions)
+			.where(eq(dailyCompletions.anonymousId, anonymousId));
+		expect(finalStats).toHaveLength(2);
+		expect(finalStats[0].anonymousId).toBe(anonymousId);
+	});
+
+	it('should handle multiple duplicates for same date correctly', async () => {
+		// Edge case: User played same date on 3+ devices
+		const device1AnonymousId = generateTestUUID();
+		const device2AnonymousId = generateTestUUID();
+		const device3AnonymousId = generateTestUUID();
+		const email = 'test@example.com';
+		
+		const testUser = await createTestUser(device1AnonymousId, email);
+		
+		const date = '2024-01-01';
+		// Three completions on same date at different times
+		await createTestCompletion(device1AnonymousId, date, 4, new Date('2024-01-01T08:00:00Z')); // Earliest
+		await createTestCompletion(device2AnonymousId, date, 2, new Date('2024-01-01T14:00:00Z')); // Middle
+		await createTestCompletion(device3AnonymousId, date, 6, new Date('2024-01-01T20:00:00Z')); // Latest
+		
+		// Migrate all to user ID
+		const user = await auth.getUserByEmail(email);
+		await db
+			.update(dailyCompletions)
+			.set({ anonymousId: user!.id })
+			.where(eq(dailyCompletions.anonymousId, device2AnonymousId));
+		await db
+			.update(dailyCompletions)
+			.set({ anonymousId: user!.id })
+			.where(eq(dailyCompletions.anonymousId, device3AnonymousId));
+		
+		// Implement deduplication
+		const allUserCompletions = await db
+			.select()
+			.from(dailyCompletions)
+			.where(eq(dailyCompletions.anonymousId, user!.id));
+
+		const dateGroups = new Map<string, typeof allUserCompletions>();
+		for (const completion of allUserCompletions) {
+			if (!dateGroups.has(completion.date)) {
+				dateGroups.set(completion.date, []);
+			}
+			dateGroups.get(completion.date)!.push(completion);
+		}
+
+		const duplicateIds: string[] = [];
+		for (const [_, completions] of dateGroups) {
+			if (completions.length > 1) {
+				completions.sort((a, b) => a.completedAt.getTime() - b.completedAt.getTime());
+				const toDelete = completions.slice(1);
+				duplicateIds.push(...toDelete.map(c => c.id));
+			}
+		}
+
+		// Delete duplicates
+		for (const id of duplicateIds) {
+			await db.delete(dailyCompletions).where(eq(dailyCompletions.id, id));
+		}
+		
+		// Verify only earliest kept
+		const finalStats = await db
+			.select()
+			.from(dailyCompletions)
+			.where(eq(dailyCompletions.anonymousId, user!.id));
+			
+		expect(finalStats).toHaveLength(1); // 2 duplicates removed
+		expect(finalStats[0].guessCount).toBe(4); // First device's score
+		expect(finalStats[0].completedAt.getTime()).toBe(new Date('2024-01-01T08:00:00Z').getTime());
+	});
+});
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