You can change 'Keep Both Files' to 'Skip' when copying multiple files in Finder.
In Lion, one of the new features is to be able to 'Keep Both Files' when copying multiple files in Finder. While this is useful there are many times when the old 'Skip' behavior is better suited to the task. Well, the (now) obvious way to do this is to just hold the 'Option' key, and then 'Keep Both File' changes to 'Skip.'
Credit for this tip goes to a poster on the Apple Discussion Boards.
[crarko adds: Sometimes simple tips are the best. This is also serves as an example of good software design.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2011102306374915