I got this strange (to me, anyway :)) build[1] error recently:

    2. Task: 13490-fedora/26, importing the package: https://copr.devel.redhat.com/tmp/tmpvGUgx7/shellfu-0.9.6-1.fc25.src.rpm
    downloading package https://copr.devel.redhat.com/tmp/tmpvGUgx7/shellfu-0.9.6-1.fc25.src.rpm
    Verifying packagage, getting  name and version.
    make sure repos exist: amahdal/rhtools/shellfu
    cmd: ['/usr/share/dist-git/setup_git_package', 'amahdal/rhtools/shellfu'], rc: 128, msg: ERROR: Package module amahdal/rhtools/shellfu already exists!

    Package already exists...continuing
    cmd: ['/usr/share/dist-git/mkbranch', u'fedora/26', 'amahdal/rhtools/shellfu'], rc: 128, msg: IGNORING: Package module amahdal/rhtools/shellfu already has a branch fedora/26

    Branch already exists...continuing
    [...]

It happened only in one of 7 chroots, while nothing in the project seems relevant to arch or distro (it's noarch, bash-related).  Re-build succeeded.

  [1]: https://copr.devel.redhat.com/coprs/amahdal/rhtools/build/13490/

So probably an one-off thing (rare race condition) but I figured I'd rather report it anyway.