So far, it seems as though everyone's is different. Granted, I presumably have not met everyone here, but of those whose talismans I have seen, none have the same symbol.

But if your theory about weakening our powers is correct, perhaps this is a reflection of that fact. Presumably none of us have identical abilities, so it's possible that a different symbol is needed to contain each of us.

[Good theory, but no. Should have compared necklaces with more people.]