Allow only one device to log in with one email - Firebase Auth

im using firebase Auth right now with email login and and the data show only with the idtoken.
its working good

but now i want to allow only one device to connect with one email adresse, so how can i do it

There may be an easy way, but a simple way would be to have all logged in users writing to your dBase with a keepalive every 30 seconds / min.

Allow auth from unlimited devices but then do a check on your keepalive variable for that user and if there is an entry within the last xx seconds then redirect to a “sorry, already logged in” page.

Maybe not as smooth as you want, but quite simple to implement.