I'm guilty I do have a few duplicates in my collection. But I don't understand why even a collector would (for example), actually buy the same game on both Xbox and PlayStation. Unless it was as a gift.
(or lot). To me that sounds like a waste of money, as the same with buying a new game that is the same as another game you already own somewhere in your collection.
Some collectors like a variety of colors, and some have already stated that they own more then one PS4 to have more saved games in the future. But that is a good reason in my opinion. But for anything else more then 2 of an earlier generation console lying around. is kinda not something I would prefer, at the moment unless, it was given to me for free.
Sometimes the game is different on a different console. but most times it is the same game. Only on a different controller and console. For example I played both "Call Of Duty Finest Hour" and "Call Of Duty 2: Big Red One on both the Gamecube and PlayStation 2 and there is not a single thing different gameplay wise, just that the PS2 version on one of them has multiplayer option and the Gamecube does not.
Sometimes it may have a new feature and thrown you an extra hambone (A new feature) to be tempted to buying, but to sound like a V8 commercial you could have bought something else and it would have been a new experience.
And sadly sometimes it cost more for the same experience on the same exact game, I don't currently understand looking back.