Hi @xiaomeng.zhang ,

Thank you for writing into the Amplitude Community! Great to see it being more and more active!

So you have two different users associated with the Device ID ending in s1y. The reason for this is due to how Amplitude determines new users and merges users. I would recommend a read of this article on the topic. 

The first event for the user was here on October the 27th with no User ID, Amplitude then gives the user the Amplitude ID ending in 493. Later on, at this event  where their User ID is now set , ending in c725, which is the same user but they now have this User ID ending in c725.

However, the other user has their first event here but it was on December the 3rd. The event was ingested from Segment so it would seem Segment sent a new User ID in with the same Device ID. Since Segment sent a new User ID with the same Device ID, Amplitude assume it is a new user, as the User ID is different. 

So this is expected behaviour. If Segment did not change the User ID and kept the same Device ID, they would then fall under the same user. Since Segment sent two different User IDs with the same Device ID, Amplitude correctly assumes it is two different user profiles sharing a device.

I would look into your Segment implementation with Amplitude to see if this is intentional implementation or a mistake. I hope this will help you with your investigations!

Kind Regards,
Denis 

Thank you! @Denis Holmes 

This is so helpful, If you need any help, please let me know.

Hi, @Denis Holmes , Did you find any problems？

HI @xiaomeng.zhang Did you reach out to Segment ? I would need you to look into your implementation with Segment. 

The first event for the user was here on October the 27th with no User ID, Amplitude then gives the user the Amplitude ID ending in 493. Later on, at this event  where their User ID is now set , ending in c725, which is the same user but they now have this User ID ending in c725.

However, the other user has their first event here but it was on December the 3rd. The event was ingested from Segment so it would seem Segment sent a new User ID in with the same Device ID. Since Segment sent a new User ID with the same Device ID, Amplitude assume it is a new user, as the User ID is different. 

So you would need to go to Segment and ask for the payloads of the events above. This way you can then see if they did send the different Device ID which would create two different users in Amplitude. It would seem the issue is on Segment’s end as it they are the ones sending two different User IDs with the same Device ID. Can you check with them for the reason they are doing this?

Thank you!

Kind Regards,
Denis