Offline Survey App - Respondent ID

I have a survey in field that are using the offline app in different devices and I have few doubts about Respondent ID.

On the Offline App:
How the Respondent ID works on the app? Is it unique on the app? Because I have one case that the same device has duplicated ID.
For different devices can create the same Respondent ID?

In the server:
After synchronizing the app, the Respondent ID can it be changed? Or we can have the Respondent ID Duplicated ?

Resolved
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