Add duplication checker

[koppor]

Refs https://github.com/JabRef/jabref/pull/5749#issuecomment-622366394

[jorgman1]

Would it not be easier to have just one list with all the abbreviations instead of one list for each subject? It would be much easier to detect duplicates, to perform changes and to check whether the journal is included.

[tdegeus]

I think having a few files is not too bad, it limits processing for most users as they work only in one or two domains.

However, a good CI tool to check for duplicates is important!! I don't think it is a problem to have the same entry occurring in several databases, but the CI tool should ensure their consistency.

In addition, I would propose to have a CSV file as follows:

name;abbreviation;variation1;variation2;...

In stead of just repeating the entry. In the latter case it is less clear what is preferred.