When I find a gun I like, I like to have two. That way, if one goes bad or something happens to it, I still have the spare. Since I like mainly older guns that are usually out of production I think it's especially important to have a spare since I won't be able to just bop down to the local shop and pick up one NIB.
I do cheat a little and don't require that the guns be EXACTLY identical. I'll let myself set them up a little differently or have both the fixed sight and adjustable sight variations of a revolver, for instance, but the controls will be the same and the gear, mags, etc will interchange.