If two unique Shadow conditions end up in play at the same time, only the first one to the FP's right is active. And the only way this can happen is if:
Player A is the FP player.
Player B plays a unique Shadow condition.
Player C would like to play one too, but he can't because Player B's condition is active.
Next turn, player B is the FP, so his condition is inactive. Player C can now play his copy of the unique condition.
When it is Player A's turn again, both of the unique conditions are now in play, but only the first one to his right is active.