It looks like that it is not possible to have sub-groups with different parents but the same name.
Looking at the tree view it makes sense to do it that way.
And I understand why this isn't (currently) possible with JabRef. It depends on how the groups are stored in the bib-file.
But the question is if you see a technical way to implement something like that?
Related bug-report: https://github.com/JabRef/jabref/issues/2695