This error comes up when you try to clone a repository in a folder which still contains .git
folder (Hidden folder).
If the earlier answers doesn't work then you can proceed with my answer. Hope it will solve your issue.
Open terminal & change the directory to the destination folder (where you want to clone).
Now type: ls -a
You may see a folder named .git
.
You have to remove that folder by the following command: rm -rf .git
Now you are ready to clone your project.