fix: prevent duplicate streak achievement notifications (#13325)

[rohithgowda18]

Fixes #13325

Problem

Users were receiving duplicate streak achievement notifications (often 2-3 identical notifications) when earning a streak milestone. Each duplicate had to be dismissed separately, causing UX friction.

Reported behavior:

• ~95% of the time, users see 2+ notifications for the same streak achievement
• Example: "streak achievement 528" appears twice with different notification IDs
• When earning multiple streaks on the same day, each can duplicate (4 notifications total)

Root Cause

The issue occurs at two levels:

1. Server-side: No deduplication logic for STREAK_ACHIEVEMENT notifications in the cleanup process
2. Client-side: Notifications tracked only by ID, so duplicates with different IDs both display

Solution

Server-Side Fix (website/server/models/userNotification.js)

• Added deduplication logic in cleanupCorruptData() function
• Uses _.uniqWith() to remove duplicate STREAK_ACHIEVEMENT notifications
• Follows existing pattern used for NEW_CHAT_MESSAGE deduplication
• Prevents duplicates from reaching the client

Client-Side Fix (website/client/src/components/notifications.vue)

• Added lastShownStreakCount property to track displayed streak achievements
• Checks user.achievements.streak before showing notification
• Provides defensive protection even if server sends duplicates
• Maintains backward compatibility with existing notification flow

Testing (test/api/unit/models/userNotification.test.js)

• Added 2 comprehensive test cases for STREAK_ACHIEVEMENT deduplication
• Ensures fix doesn't affect other notification types
• Validates existing NEW_CHAT_MESSAGE deduplication still works

Changes

• Modified: website/server/models/userNotification.js - Server-side deduplication
• Modified: website/client/src/components/notifications.vue - Client-side duplicate prevention
• Modified: test/api/unit/models/userNotification.test.js - Added unit tests
• Added: DUPLICATE_STREAK_FIX.md - Implementation documentation

Testing Instructions

1. Run unit tests: npm test test/api/unit/models/userNotification.test.js
2. Manual testing:
   • Complete a daily task that achieves a 21-day streak milestone
   • Verify only ONE notification appears
   • Check notification can be dismissed with single click

Impact

• ✅ Prevents duplicate notifications at both server and client level
• ✅ No breaking changes to existing notification system
• ✅ Minimal performance impact (uses existing cleanup process)
• ✅ Backward compatible with older notification data

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