To clarify, different users should have different IDs, please make sure each user's ID is unique.
Does user mean client? If so, PlayFab currently doesn't provide an API to invalidate client session tickets, if this feature is what you want, you can vote for it. If not, feel free to post a feature request.
If what you need is to prevent duplicate logins, you can refer to the solution in How to prevent duplicate login? - Playfab Community.
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