My original suggestion was to request that back links was added to the original question. But that had been discussed a lot already.
The problem today is that duplicates can contain better answers than the original question. And without some kind of back link or modification of the original question it can be quite easy to miss the best possible solution to a problem.
In one of the questions (here @meta) I saw that the Flag question feature could be used to request that answers should be merged with the original question. I was not aware of that. I though that the flag feature was only to report some kind of abuse.
I suggest that another link is added to
link|edit|flag that are available for each answer of a question when the question is closed:
link|edit|flag|move to original question. (Requires 5 votes to be moved)
Well I'm not sure how it should work, all I'm sure of is that (almost) nobody knows that you can request that answers should be merged with the original question.