If the answer goes into detail regarding an edge case brought up in another answer, I do not see any problem.
From what you have established, the new answer goes into more detail on a particular issue that was only mentioned in the original. Perhaps the original poster thought it was not worth going into detail, for the minority of the cases that would need to take the extra considerations, or progress with the more particular answer. Regardless, it is useful to have a detailed answer regarding these situations, for users that do find themselves in this minority.
If the additions are semantical, I would favour downvoting. If not, take your own judgement into consideration, in regards to voting.
As a whole, there is no real problem, here.