In order to mirror a git repository from one remote to another, first clone it with --mirror:

git clone --mirror <repo.git>

(This will leave a bare copy of the repository, containing an internal directory structure with the object.)

Then add the new remote (let’s call it mirror-remote) which will hold the mirror:

git remote add mirror-remote <mirror-repo.git>

Now push all the stuff to the mirror-remote:

git push --mirror mirror-remote

That’s all! Now every time you want to update the mirror just fetch the new stuff and push it:

git fetch
git push --mirror mirror-remote