why a user have two device_id

@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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