How can I get the player's online / offline status. The problem I have right now is that an account can log in at the same time.
PlayFab is a stateless service, any clients can make API calls as long as its session ticket is still valid, hence it’s hard to tell whether the players are online or offline, this is also one of the reasons why PlayFab does not provide such API to get player’s online/offline status. Moreover, Login APIs are anonymous, which means that any clients can call them as long as they have valid credentials (username/password, 3rd party platform tokens, etc. ) and use the returned session ticket to make successive API calls, acting like these clients are all “logged-in”, this is the reason why an account can “log in” on multiple clients at the same time. I think the problem you are having can convert to the problem of how to prevent duplicate logins, about this, we have a fairly complete discussion here: How to prevent duplicate login?
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