When Joel wakes up from the procedure, he simply goes back on the train and meets Clem again.
A do-over will not make one a new person, but simply an ignorant one destined to repeat themselves until the cycle breaks.
And you’ll notice the pair have the same conversations a few times over, denoting multiple timelines where they could have been together in the past twice.
But if we have not learned, we are destined to repeat ourselves over and over.