I don't know for sure, but at least for me my chances of checking a question drop significantly when it already has an answer (and will continue to drop as the question gets more answers). The problem is that this leads to some questions not getting good answers because someone who actually knows the answer won't even look at it because it seems like it has already been answered.
I think it would make sense for a question to either appear unanswered (probably too misleading) or at least make it clear that all answers to the question have 0 or perhaps -1 or less votes. This will help to increase the chances of someone else viewing the question and providing a better or real answers.
There are times when I ask a question and I check back and it has an answer, but the user did not understand or properly read the question. I notice that these questions rarely get other answers, but I can't signal anyone that the answer I got is no good and I need more help. Duplicating the question is no good either.