iMovie '09 stores duplicated clips

I copied a 3-minute event (1.38GB) in iMovie '09 stored on the hard drive of my PowerPC G5 to an external hard drive and then from there I imported it to iMovie '09 to my MacBook Pro. I noticed that all clips were now shown twice in iMovie, so I deleted one set and now the same 3-minute event only takes up 612MB on the MacBook Pro. Could anyone tell me why imovie had originally stored the clips twice and what I can do to avoid having duplicate clips on my PowerPC G5 (taking twice the space on the hard drive) that I have to delete after I transfer them to iMovie on another Mac?