Do you mean the duplicates that iPhoto makes when modifying an image? Or duplicates that you have accidentally imported twice?
There are ways for the former I think - scripts and apps that will do it. But unless you are REALLY tight for space, why?
I understand people don't like 'wasted' space, but the reason I have large hard drives is to increase functionality. Having non-destructible editing is a functionality that is worth that extra space, in my opinion.