To do backup on existing git repo please follow steps below.
Login to master machine.
Considering you already have key based access between master and backup server.

git remote -v

cd $HOME
git --bare init --shared test.git

after you taste your login.

cd ~/test.git
git remote add backups --mirror git@backup.k2patel.com:test.git
git push backups
cat .git/config

Result of last line should should contain text like the next block.

# [remote "Backup"]
#   url = git@backup.k2patel.com:test.git
#   fetch = +refs/*:refs/*
#   mirror = true

#!/bin/bash
#Build by. kpatel
# k2patel@hotmail.com
 
## Nightly Backup of Repository.
 
## Variables -- ( change According to your need )
repohome=(/home/backup/test.git /home/backup/test2.git )
 
export receipients="k2patel@sify.com k2patel@hotmail.com"
 
## Static 
export git=$(which git)
 
## Methods 
notify(){
    for n in $receipients; do
        echo $1 | /bin/mail -s "[GIT] Backup $k2  Failed" $n
    done
}
 
## Programm
for k2 in ${repohome[@]}; do
    echo $k2
    if cd $k2 && $git push backup ; then
        echo "done"
    else
        notify "GIT Backup Failed, $k2"
    fi
done