I'm not sure I get the "one person playing 2 characters" push back/irritation. That seems to me to be the least of the issues here...assuming a player wants to play 2 characters and is experienced/"good" enough as a player to make that work. In this case, it doesn't sound like they do want to play 2 PCs...so I would drop one and give the other to the DM to NPC or "write out" of the story as they see fit.
It's certainly not necessary to play 2 PCs (regardless of what a party "needs"/doesn't have niche-wise). But if it happens, I don't understand why that would matter to anyone else at the table. More characters = more chances for success...No? Or how someone have 1 PC but still running a secondary follower/"NPC" [which, I would submit, the DM would/should be controlling anyway] is any different than someone running 2 PCs.
I mean, it certainly wasn't a common occurrence bitd, most players definitely had their single/primary PC that was their favorite/they loved to play in a given campaign and that's all they played. But it happens/happened. Especially with somewhat flexible/amorphous groups with players coming and going.
Any insights there for a slightly confused 'Dragon? Is this another "spotlight/special snowflake" issue that I just don't get/have experience with?