Hi @xiaomeng.zhang,

This is Yuanyuan from the Amplitude Support team - thank you for writing this post!

The two users you shared are both anonymous users and they have two different Device IDs. In this case, they will be considered two individual users. Amplitude merges users based on their Device ID and User ID. When the same Device ID is detected for anonymous users, Amplitude will merge anonymous users or merge an anonymous user to an existing user. When a user is an existing user with a User ID, and they log in on different devices, Amplitude will merge the events on multiple devices into one profile. Please check this article for detailed information about how Amplitude tracks unique users and merges users. 

I noticed the two example users have a custom property ‘user_id’ and they both have the value 193. But Amplitude will not merge users based on their custom properties. I suggest modifying your instrumentation and assigning Amplitude User IDs when the users can be identified. This way, users with the same Amplitude User IDs on different devices will be merged. 

Hope this helps!

Best,

@Yuanyuan Zhang 

Hi, Yuanyuan

Although the device id is different, he is actually a person, the mobile phone is the same, but there are two device ids.

This happened after I uninstalled the app and re-downloaded it

Hi @xiaomeng.zhang, 

Device IDs will change after the user uninstalls and reinstalls the app because it is a random alphanumeric string generated by Amplitude. This string is stored locally in the mobile device and after the app reinstall, a new string / a device ID will be generated. Same as the property ‘‘user_id’’, custom properties cannot be used for identifying and merging users at Amplitude. 

If you would like to avoid this issue, you can either pulls the Advertising ID as Device ID or assign Amplitude User IDs to users after they sign up. 

Please check the article for how Amplitude tracks unique users in details: https://help.amplitude.com/hc/en-us/articles/115003135607. 

Thanks for this interesting information!

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