Unless you're pretty sure you have duplicate entities, there are other ways to minimize file size. With overkill you can select options to *not8 combine collinear or overlapping lines and arcs, and whether to ignore color, linestyle, layer, etc in comparing objects. Did you read E.T. help?
Have you used purge, layer filter delete, etc. as first methods of reducing file sizes? Unless you have lots of duplicate objects, these methods will likely do a lot more for reducing file size than Overkill will.