I have duplicates of the ones I carry the most because I tend to lose or break them. However, I always try to make the duplicate slightly different. For example, one of the knives I carry the most is a Spyderco Delica. The one I carry is the standard, saber ground, black FRN handle, plain edge Delica. My "duplicate" is a British Racing Green, combo edge in ZDP-189. I keep a Ladybug 3 on my keychain. It is an orange FRN handle, VG-10 blade. I have a duplicate in case I lose it (actually did lose it, but found it again recently), but the "duplicate" is the yellow Salt version with H1 steel. The knife I rotate out with the Delica is a Benchmade Mini Griptilian. I only have one of them, but I am thinking about getting a duplicate either in another color, or maybe the Doug Ritter version in the upgraded steel. At least by getting slight variations I don't feel like I'm wasting my money and not adding anything to my collection, but I still have a backup in case one of my favorites gets lost.