I have duplicates, but only rimfire for some reason.
2 Ruger mkIIIs, one for me, one for the Mrs ... they have the same up/down-grades but things are set a bit different for individual taste.
3 Ruger 10/22 rifles, all with Tech-Sights and sling setups. One is a M1 Carbine trainer (mine), one is mostly stock (MrsBFD's), and the newest is the International model with the slightly better TS, I plan on using it for longer range stuff and it was MrdBFD's Xmas gift to me.
The Why? part is simple, they share magazines, tools, obviously ammo, and parts, I have a bag of 10/22 stuff and a pistol case of mkIII stuff, including mag loaders for each, at this point if I were to desire another semi-auto .22lr it would seem foolish to get away from the existing pile of accessories.
For other stuff, I haven't found anything that screams to be duplicated, but I do try to have two guns in any given caliber ... right now I'm trying to figure out another .45acp gun, I may end up with a duplicate Officer's Size 1911, or a CCO if I can find one for a reasonable price, just to have something a bit different.