Looking at my stash and completed models, my multiples fixation is WWII Jeeps.
Currently, I have two Tamiya Jeeps, one built out of box, and one converted to a British SAS desert jeep, using spare parts, figures jerry cans and weapons from a broken DML SAS jeep kit. I also have a DML SAS jeep kit that survived intact and was completed. Add to that two Italeri ambulance jeeps, one from about 20 years ago, and the other the D-day anniversary re-pop.
And finally, a Tamiya Ford GPA amphibious jeep, built out of box, for a total of six jeeps.
I think it's mostly because I don't have a lot of shelf space for kits, and at 1/35 scale, the jeeps have plenty of detail and are interesting to look at.
Outside of that, the other multiple model that sticks out is the Starship Enterprise kits in my stash. Two Enterprise Refit models in 1/1000 scale, two Original Series models in 1/1000 scale, and 1 big, honkin' 1/350 scale Refit kit.
Great question.
Randy