Seems like with 35mm slides, doubling up would give you one (#1) to project and/or throw on a light table (and one may require a little different exposure than the other use, but there could be eventual fading) and one (#2) to keep safe (for scanning or making dupes -- after scans were made of #1.) I assume making 'dupes' in-camera would be the cheapest and highest quality way to go.