I have duplicates of sever of my favorite guns. And by duplicate I mean same caliber and barrel length. They can wear different grips. I don't consider the same model in a different caliber to be a duplicate.
I have them because I wanted them, and had the money at the time to get them. They keep quite well. I'm not concerned about wearing out my favorite, but things do happen, things can break, and it is comforting to me to know that if something puts my main gun out of action, I have another to use until my regular is back up and running.
Also there is the (unlikely) defense use. If you ever do use a firearm defensively, expect the police to take it (evidence), and keep it until ALL legal processes are completed. Having a duplicate means you aren't without until you get it back from the police, and also since it is a duplicate, you don't need to re-train on something different.
And, there is also the potential that you can sell the duplicate(s) at need to fund other things. I used to have four 6" Highway Patrolman, now, I'm down to two. I could part with one more, at need, and still have one, though I'd prefer not to.
If there is a gun you really, really like, and you can afford it, get a second one. You don't need to use it, unless you want to, you could consider it a "savings bond" and put it away until needed. And, there is also the fact that many guns, or specific variations of them, DO go out of production, so you can't count on being able to get an exact replacement for you favorite years down the road when you actually might need one.