I have two children and would like one of each to pass on to them. That justifies the first two of any model ("Honey, it's for the chiiildren"). Somebody else might want 4 M1 Garands, one of each flavor. Duplicates of a carry gun for reasons mentioned, but then the preferred carry gun changes and about the time you make the switch you figure you ought to get a second copy of the new gun. The old carry wasn't bad, it's just not your favorite right now so you may figure to hold onto them (for now). Guns aren't getting any cheaper, after all.
I don't do the shotgun sports, and can't really hit much in the air with a shotgun, but I'm getting ready to buy a second pump 12g shotgun so that my son can graduate from Mom's 20g. I'm left handed, and prefer Mossberg over the 870. Despite the fact that my son is right handed and despite the glut of used 870s for reasonable prices, I'm thinking of getting a second Mossberg 500 so that the controls are all in the same places in case I want to use "his". His deer rifle is a Savage, so the tang mounted safety would be better for him (I'm not sure how I feel about the compensated barrels on the newer Mossbergs I would have already bough it).
Errh,
Why not?