<s>Another idea: why not downvote any "extra" answers to duplicate, closed questions? If the only thing certain users respond to is votes, then maybe a few downvotes are in order? Or, perhaps disallow voting on closed questions entirely?</s>

After looking at the examples Diago cited, **I am not sure there is a problem.** 

>  1. [This](http://superuser.com/questions/103961/build-quality-of-laptops/103966#103966)
> answer on a question closed as
> subjective by the community caused
> some interesting comments.
>  2. [This](http://superuser.com/questions/104956/nda-and-contract-template-for-uk-closed)
> question which is not computer related
> has 1 answer. Common pattern here:
> Vote to close and answer question. 
>  3. [This](http://superuser.com/questions/104641/what-is-your-favourite-file-copy-manager-replacement-for-windows-closed)
> exact duplicate generated 1 high rep
> answer.
>  4. [This](http://superuser.com/questions/104080/what-sorts-of-things-can-i-do-with-ssh-on-the-iphone-closed)
> question should have been closed
> outright.
>  5. [This](http://superuser.com/questions/103934/free-modeling-tool-closed)
> is another multiple duplicate
> question.

None of these closed questions resulted in any significant rep, and the answers did not seem harmful.

The one exception is the cut-and-pasted duplicate answer on #5. In this case, I think the (answer to the) closed question *should be deleted*, primarily **because of the blatant cut-and-paste rep gain**, which means any rep gained would be removed on recalc as well.