bugreport: "git checkout -B" allows checking out one branch across multiple worktrees

[Jinwook Jeong <vustthat@gmail.com>]

What did you do before the bug happened? (Steps to reproduce your issue)

1. `cd` into any git repo that has at least one commit.
2. Identify the current branch, say main
3. $ git branch foo # a new branch
4. $ git worktree add ../new_worktree foo
5. $ cd ../new_worktree
6. $ git checkout -B master HEAD


What did you expect to happen? (Expected behavior)

git prints out "Fatal: 'main' is already checked out at ..." (or
similar message that informs me that I'm not allowed to doubly
checkout one branch)


What happened instead? (Actual behavior)

The new_worktree successfully changes its current branch to master,
which is already held by another worktree.


Anything else you want to add:

https://www.git-scm.com/docs/git-checkout#Documentation/git-checkout.txt-emgitcheckoutem-b-Bltnew-branchgtltstart-pointgt

According to the documentation, "git checkout -B BRANCH START" is the
transactionally equivalent of:

  git branch -f BRANCH START
  git checkout BRANCH

When I ran the first command in place of the step 6 of the above
reproducing procedure, git refused to carry on;
I suppose that this is the intended behavior for "git checkout -B".


[System Info]
git version:
git version 2.39.0
cpu: arm64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
feature: fsmonitor--daemon
uname: Darwin 21.6.0 Darwin Kernel Version 21.6.0: Sun Nov  6 23:31:13
PST 2022; root:xnu-8020.240.14~1/RELEASE_ARM64_T6000 arm64
compiler info: clang: 14.0.0 (clang-1400.0.29.202)
libc info: no libc information available
$SHELL (typically, interactive shell): /opt/homebrew/bin/bash


[Enabled Hooks]
not run from a git repository - no hooks to show