You shouldn't. If you open a treasure chest that contains a Gambit, the game will automatically replace the other possible item that you can get, usually gil. If in the instance that you DID get the same Gambit twice, it's possible that you misread the Gambit name (many of them are incredibly similar), or that you got it twice anyways, not that it affects the game in any way whatsoever.

It only means you got cheated out of the possible maximum gil that you would have gotten from that chest, plus the X multiple of 50 for the cost of the Gambit. At most, I'd say a loss of 125-700 gil.

Small problem, not-so-hilarious results.