I started a new campaign for my D&D 5e group and we had made some excellent progress and the characters had made it to 4th level. While entering the "Stone Tooth" through the front door one of the players had their character advance across the rope bridge over the deep chasm. The character was hit by a thrown spear and then the player rolled a natural 1 on their saving throw to keep their grip on the bridge. The character was then irretrievably dead since it fell 200 ft into the underground river.
The party decided that going through the front door had been a bad idea and retreated back to the nearby town so the player could roll up a new character. The player rolled up a rather different character and we played on and the party made it further into the adventure using another entrance.
Later the player of the dead character realized that they weren't having as much fun with their new character and asked if there was anyway they could get their old character back. I came up with an in game way for this to happen that added some interesting role playing difficulties and set the new character against the returning old character.
However, I am not sure how to have these two characters being played by a single player be adversaries. I have thought of taking over the new PC and have the player only play the old character but I'm sure if this is such a good idea. I also don't want this player playing more than one character, I prefer that the players focus on a single character.
I have 5 players and they are powerful enough that they can handle the level appropriate encounters, adding a 6th character is going to over power the party and I worry that some players might be jealous of this player having two characters. I want everyone to have fun!
What should I do?