I wouldn't want to completely block duplicate answers. Duplicate answers most generally indicate a problem with the post, rather than the user.
While keeping the current behavior (duplicate answers to be flagged), I propose showing the user with a notice, something similar to
Wait!
Was this question asked before? We noticed that the answer you gave to this question is exactly the same as [this other answer you gave to a different question](link to his other answer). Are the two questions the same?
If they are, please consider flagging one of the questions as duplicate of the other! This helps us keep the site clean and your original answer easier to find!
If the questions aren't the same, please consider editing your answer to match this specific question.
The bet here is that the user's workflow is interrupted and so he'll at least absorb some of the message. If the user opts to submit his answer regardless, the same flag should still be raised.
