[BUG] fetching all remote branches results in failed multiple updates

Hello all,

When running the latest release of git, I receive an error when attempting to fetch all remote branches from a repo:

    fatal: multiple updates for ref 'refs/remotes/origin/maint' not allowed

The specific ref that it fails on changes depending on the repository, but the end result is the repo isn't cloned.

The specific configuration that causes this bug is the `remote.origin.fetch` option, specifically:

    [remote "origin"]
    	fetch = +refs/heads/*:refs/remotes/origin/*

These settings are listed as an example in "CONFIGURED REMOTE-TRACKING BRANCHES" of git-fetch(1), as well as expanded upon in https://stackoverflow.com/a/40739835.

I'm running git version 2.21.0.

Full reproduction steps are below:

    [root@chabuduo ~]# useradd -m git-test -s /bin/bash
    [root@chabuduo ~]# sudo --preserve-env=SSH_AUTH_SOCK -u git-test -i
    [git-test@chabuduo ~]$ git --version
    git version 2.21.0
    [git-test@chabuduo ~]$ cat > ~/.gitconfig
    [remote "origin"]
    	fetch = +refs/heads/*:refs/remotes/origin/*
    [git-test@chabuduo ~]$ cat ~/.gitconfig
    [remote "origin"]
    	fetch = +refs/heads/*:refs/remotes/origin/*
    [git-test@chabuduo ~]$ git clone git@github.com:git/git.git
    Cloning into 'git'...
    Warning: Permanently added the RSA host key for IP address '192.30.255.112' to the list of known hosts.
    remote: Enumerating objects: 265113, done.
    remote: Total 265113 (delta 0), reused 0 (delta 0), pack-reused 265113
    Receiving objects: 100% (265113/265113), 113.20 MiB | 5.49 MiB/s, done.
    Resolving deltas: 100% (196542/196542), done.
    fatal: multiple updates for ref 'refs/remotes/origin/maint' not allowed