By accident, a branch remotes/upstream/upstream/develop has been created. Upstream is under GitHub control. The standard delete command fails:
$ git fetch --all
$ git branch -a
* develop
master
remotes/origin/HEAD -> origin/develop
remotes/origin/develop
remotes/origin/master
remotes/upstream/develop
remotes/upstream/master
remotes/upstream/upstream/develop
$ git push upstream --delete upstream/develop
error: unable to delete 'upstream/develop': remote ref does not exist
error: failed to push some refs to 'git@github.com:...'
.git/config has the standard stanza
[remote "upstream"]
url = git@github.com:...
fetch = +refs/heads/*:refs/remotes/upstream/*