I just discovered that if I try to add the answer using only the question portion of the url:
It works correctly. I'm not sure how the systems decides which answer to pick if you've answered the same question twice, but I don't think that will ever cause me any issues.
Since the system expects only the url to a question, that should probably be made more clear, or it should automatically strip off the end of the url to determine which question you mean.
An interesting thing to note is that the error message changes slightly depending on the url:
Both give me:
The link you provided does not lead to a stackexchange answer by you. Please try again.
But if I strip a few more characters off:
I get a different error message:
The link you provided does not lead to a valid StackExchange answer. Please try again.
Any url between these two seems to work fine: