This is the kind of shit that SHOULD NOT happen. If fare enforcement is in effect, focus on people actively seeking to avoid paying a fare. I have also seen situations where a turnstile is slow to respond to a tap and the person ends up tapping twice, paying double fare and then the turnstile stays free for the next person. Should the next person that uses that turnstile with a credit in it be ticketed for not paying? The fare got paid by someone else, so it shouldn't be a problem.