In my experience, when there is a mistake in the question, I have found that it is usually best to inform the students and give everyone credit for the problem. Tell them they can resubmit their answers to see if they really got it right or not, and continue to resubmit until it is right. But it is not worth trying to force those who thought they got it right to do it again. They will just be resentful and will not learn anything from it, and you will come off looking like a Scrooge. The score for one problem is not worth the headache for everyone. My two cents worth.