You need those repositories:
git clone https://github.com/freifunk-kiel/fastd-git-hook
git clone git://git.freifunk.in-kiel.de/fastd-peer-keys.git
Add the file pre-commit to your .git/hooks/ folder:
cp fastd-git-hook/git/hooks/pre-commit fastd-peer-keys/.git/hooks/pre-commit
And make it executable
chmod +x fastd-peer-keys/.git/hooks/pre-commit
Now whenever you commit a new key, there is a check if the syntax is ok and if there are no double keys.
Additionally, if you only use small letter key-files, you can uncomment the part at the end that renames all capital letters:
for f in *; do
NEW="$(echo $f | tr '[A-Z]' '[a-z]')";
if [ "$f" != "$NEW" ]; then
git mv -v $f $NEW;
fi;
done;