I've been using a dupe rig with my digital cameras for years. First I was using it with my EOS XS, a 10.1mp DSLR. Now I'm using my 24.3mp NEX 7, which gives me 6000x4000 pixel images, same as a Nikon Coolscan. At its core is a 55mm f/3.5 Micro Nikkor, an exceptionally sharp macro lens. Threaded into the 52mm threads on the front of the lens is an old Opteka "digital slide duplicator" tube. I stripped everything else off the tube, including the internal element that allowed for close focusing, so that it's just the tube with the flange on the end, which I use to slide the duplicator attachments onto. One of the nice things about this tube is it's two-piece and can be rotated, which is handy for making sure that the image sits straight. To duplicate my setup, you'll need to scrounge up one of the Opteka digital slide duplicators, which might not be such an easy thing to do. Current models are different from mine and I don't know how well they would work. Anyway, I paid about $55 for the Opteka, where I eventually stripped everything off of it. Kind of expensive for just that tube, but I don't know of an easy substitute to recommend. Here are some detailed pix of my rigs.
This is what I used with my APS-C EOS DSLR:
This is the rig I use with my NEX 7:
I also have a roll-film stage that I use for duping negatives. Both the slide stage and roll film stage have metal clips that fit on the flange of the Opteka tube, allowing the stages to slide on, and be positioned up and down. With the dupe arrangement for the EOS, I'm getting about 80% coverage of slides and negatives. In another words, there's a thin border all the way around the slide image -- it doesn't quite fill the frame. With the NEX7, coverage is slightly more than 100%, meaning the image is cropped, but by a very minor amount. Less than 2% of the image is cropped out, I estimate.
This is the stripped down Opteka tube:
And the slide stage, which I cannibalized from a Cambron zoom slide duplicator, which I bought off eBay for $8:
I kept my eyes peeled for a roll-film stage on eBay and it wasn't too long before one turned up. I think I had to pay about $10 for it. Here's a shot of the dupe rig, showing the roll film stage on the right of the photo.
Here are a couple of images I shot with the dupe outfit and my NEX 7. This first one is a Kodachrome 64 slide:
This one's a Fujichrome 100 slide, circa 1986: