In the git help pages it says

The name of a new directory to clone into. The “humanish” part of the source repository is used if no directory is explicitly given (repo for /path/to/repo.git and foo for host.xz:foo/.git). Cloning into an existing directory is only allowed if the directory is empty.

So if you do need to do this, then a work around is:

Clone the repository without checking out the contents to a new folder

git clone -n <repo>

Then move the git folder to where it needs to be

mv .git <existing folder>

Then force a check out the contents

git checkout -f master

Warning: This come with no guarantee of success. Works on my machine…