Git fast-import : Warning duplicated ref: refs/remotes/origin/master

Hi,
I am currently working on the git-mediawiki gateway project.
This project uses fast-import to get data from mediawiki. For each
revision from mediawiki, we generate a chunk of fast-export stream
looking like this :

commit refs/remotes/origin/master
mark :<int>
commiter <user> <address> <timestamp> +0000
data <sizeofcomment>
<comment>
M 644 inline <title>.wiki
data <sizeoffile>
<content>

At the end of a git clone mediawiki::http://ourwikitest, everything
looks perfectly fine, including the git log, but a "Warning duplicated
ref: refs/remotes/origin/master" is thrown. And indeed, it is
duplicated in .git/packedrefs

A git gc solves everything, but I wondered if there was a way to fix
this or if this was a known bug.

Thank you,
Regards,

--
Jérémie Nikaes