Add duplicate-file-open prevention ability

[GavinRay97]

Note that this will not work properly if files are opened before the plugin is initialized as it cannot track them.

[sum01]

It's not a bad idea, but this won't work if a file is opened in a way other than using the filemanager.

People might be annoyed at this type of change without it being behind an option (although I personally don't care). It's minor, but someone might open 1 file side-by-side, which this doesn't allow for.