Duplicate paths should not be allowed

[jhiesey]

What version of this package are you using?
4.7.0

What operating system, Node.js, and npm version?

What happened?

When createTorrent is passed an array of files, where multiple files have the exact same name or fullPath values, it will create a torrent containing multiple identical paths.

Such a torrent with duplicate paths in it is not valid, so createTorrent should call the callback with an error.

What did you expect to happen?

Are you willing to submit a pull request to fix this bug?