Thank you! Yes, it works:)

Thanks a lot for the response! I tried to replace d[player].add(group) with d[player].update(group) as you suggested in the other comment, but it still does not work for this input.

Please check this input:
s = [
["AB", "AC","DE"],
["AD", "AE", "BC"]]

The function returns true, though it should be false, since A is playing twice each day. Thank you!