Add rex_firebase_singleLogin plugin, which could be used to test if account had been login (online) more than one times.
First you might run parse login,
after login, get userID by Expression:UserID in parse plugin, and use it with this rex_firebase_singleLogin.
If "Condition:On success" (rex_firebase_singleLogin) had been triggered, it is the first time to login this account, otherwise "Condition:On error" will be triggered. i.e. mutiple login at the same account (userID).