Git won’t let you check out the same branch twice, because if you do, and then go to one of the two work-trees and make a new commit, you’ll set yourself up for misery when you go back to the other work-tree.
If you have actually removed the other work-tree, simply run git worktree prune to make Git realize this. If you have not actually removed the other work-tree, don’t check it out twice: it’s no fun.