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A check is a contract of payment between you and the payee (in this instance, John). When John convinced Gary to cash the check, it does not obligate you to make sure that payment is good as to Gary. That is Gary's problem, as Gary in fact cashed a faulty check on which you stopped payment. You did not violate any law by stopping payment, as John had for all intents and purposes disappeared.
So, you are not in anyway obligated to Gary and Gary does not have any recourse against you. If you go and pay Gary, you are doing so to pay him for John. The stopped check should not come back to haunt you. However, I suggest telling John that you are not going to do this for him and that he can handle paying Gary. Why should you?
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