No 404 code returned

Hi,

For some reason Google has spidered some weird urls from my site which uses sef advance.  For example ' www.mysite.com/?action=login ' is a url that appears in google's index.

The problem is that when following this url it takes you to my home page with the code '200 ok' returned.  This url should return a 404 error code no?

This problem has led to duplicate content issues for my site in Google's index.

Is there a solution to  this problem?

Cheers

That's not really SEF Advance problem. More of a Joomla! one. Even with SEF turned off such link would return 200.
It would be impossible to fix on SEF level since if such links would be blocked no arguments would be allowed to be passed to the site which would create lot of other problems.