worktree add already exists

x@y MINGW64 ~/gittest $ ls bare/  barework/

x@y MINGW64 ~/gittest $ ls -a barework ./  ../  test.txt

x@y MINGW64 ~/gittest
$ git -C bare branch
* master

x@y MINGW64 ~/gittest $ git -C bare worktree add --no-checkout ../barework Preparing worktree (new branch 'barework') fatal: '../barework' already exists

x@y MINGW64 ~/gittest
$ git -C bare branch
   barework
* master

Why this doesn't just work and if not why is barework branch made then, why at all ?

I would like to attach an existing dir to git (make it a workdir) and then update the index with git reset and checkin the differences.

I can do that tricking around with a little dir renaming, new empty dir, move the .git file and rename back...