Upon install of the app, check the clusterstorage for a unique user id.
If no unique user_id is available, then contact the server. The server creates a unique id and sends it back. store that unique id in the clusterstorage.
On the server side, You can also use their IP address to see if you already know the user. Of course, people sharing the wifi get the same user id.
Then make sure people validate their account. You provide them a unique code (6 digits). Let them enter that on a website with their email. Once you see the email was already registered, merge the accounts and set the correct user_id back to the clusterstorage.
Check how payment provider kiezelpay does this. It basically needs to check for a user-app combination if it was purchased or not.
I also send data from various device to my webserver.
