iPhoto and Aperture can use the same library. When you edit a photo in iPhoto, it duplicates the photo and keeps the original. Aperture on the other hand saves only the edits and not a new file. What happens to these 'originals' when I open a library that was only used with iPhoto in Aperture? Can I remove the iPhoto 'originals' or are they kept even though Aperture handles things differently?
See this link for how Aperture handles master images.
Masters for images that you import as referenced images may reside in many different locations on your hard disk storage devices instead of in the single location of your main Aperture Library file.