Cloning and Forking a git repository is a two different approach involved managing the source code of a repository.

Git Fork is a process of creating a copy of repository whom want to fork, and keep it separate over the central repository system. 

Any changes to parent repository will also get reflect into the forked repository, but if want to update the changes upstream from forked repository to main repository, then we require to create a pull request.

Git Clone is a process of creating a copy of repository into our local system. This will be part of the central repository, where multiple developer can work and push their changes to the repository. 

Git clone not create any duplicate repository on the central repository system.