Sorry if i am repeating a question here (could not find answer).
I am playing around with challenges for online card game and i note sure i understand, upon a user login, how to detect if a player is already joined in any other challenges.
There is no out of the box method for doing this I'm afraid. What you could do is store active challengeIds in a players scriptData. When they join a challenge, add it to the array of challenges. When they leave the challenge or the challenge ends remove the ids. You can then load this array on authentication to see what active challenges the player is in.