That's not entirely true though.
We SHOULD be able to select different skills when we already have the ones given by our background (I'm not sure why Larian chose to deny us that). Then there's the OPTIONAL rule that a player, with the DM's consent, can change skills or tool proficiencies to better fit the character (along with other background features, but those aren't part of BG 3 anyway) or the campaign world.
I'd love to see duplicate skills handled according to the PHB, but I somehow doubt that will happen.
Custom Backgrounds arent an optional rule, I literally just explained this.
Its a completely rules as written rule, that does not need the DM's consent (at least not more than anything else in teh game)
skills for your background can be any two skills
https://online.anyflip.com/dkneq/yerq/files/mobile/126.jpg
