Amplitude User ID and Device ID Not Merging
Workaround available Web, iOS, Android Updated November 13, 2026Based on 2 user reportsHow we research
Quick answer
Amplitude cannot merge two different user IDs, and anonymous events only merge when a device links to a second Amplitude ID before sign-in. Send a stable user ID at login and reset on logout.
Most likely fix: Set a stable user ID
How to fix it
Try these in order — the most common fix is first.
1. Set a stable user ID
- Call setUserId with the same ID as soon as the user authenticates
- Do not assign user IDs to anonymous users
- Use the same user ID variable in every project in the organization
Did this fix it?2. Reset on logout
- Set the user ID to null and regenerate the device ID using your platform's SDK reset
- Without it anonymous events after logout count toward the last user
Did this fix it?3. Inspect a user
- Open User Activity to see the merged Amplitude IDs and merge times
Did this fix it?
Symptoms
The same person counts as several users, or two people on one device are merged.
Why it happens
- User got a new user ID (IDs are never merged)
- Different user ID variables across projects
- No reset on logout, so later events count toward the previous user
- Reinstall creates a new device ID
This guide also covers
- amplitude user id device id not merging
- amplitude duplicate users
- amplitude users counted twice
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