JabRef version 5.2, 5.3, master commit 049acb9 on Linuxmint-20.1.

JabRef 100.0.0
Linux 5.8.0-45-generic amd64
Java 14.0.2
JavaFX 16+8

• Mandatory: I have tested the latest development version from http://builds.jabref.org/master/ and the behaviour persists

Duplicated groups in different branches of the group tree are not detected when a database that contains them is read in.

But on a second thought, and after playing a bit with JabRef 5.x master branch, I start thinking that this is not a bug but actually a very useful feature! I've opened a detailed discussion in https://discourse.jabref.org/t/hierarchical-groups-with-duplicated-names-are-actually-working-in-jabref-5-x/2619. So don't fix it!

Steps to reproduce the behavior:

1. Start a new JabRef process, create a new library, add an entry, save the database as 'new.bib' (see the attached 'biblio.zip');
2. Show the groups interface, create two groups 'Human' and 'Computer', create a subgroup 'Languages' in 'Human', save over the 'new.bib';
3. Assign the new entry to 'Human/Languages';
4. Copy 'new.bib' to 'new-duplicate-group.bib';
5. Open the 'new-duplicate-group.bib' with a text editor, and duplicate the 'Languages' group in the 'Computer' branch:

saulius@starta duplicate-groups-created-by-hand-are-not-detected/ $ diff -u new.bib new-duplicate-group.bib 
--- new.bib	2021-03-20 14:13:07.189182296 +0200
+++ new-duplicate-group.bib	2021-03-20 13:35:47.348262111 +0200
@@ -17,4 +17,5 @@
 1 StaticGroup:Human\;2\;1\;0x8a8a8aff\;\;\;;
 2 StaticGroup:Languages\;0\;1\;0x8a8a8aff\;\;\;;
 1 StaticGroup:Computer\;0\;1\;0x8a8a8aff\;\;\;;
+2 StaticGroup:Languages\;0\;1\;0x6a7a9aff\;\;\;;
 }

The resulting BibTeX files are attached in 'biblio.zip':
biblio.zip

6. Load the edited file into JabRef; no error report is produced, both groups are shown (correctly?), even though the GUI would not allow me to create such situation; the entry is reported to belong to both groups:
   

The manually created duplicate is used only to reproduce a minimal example of this behaviour; in real life large number of duplicates emerged when porting previous group trees from the previous revisions of my database.

This behaviour actually seems to be very useful, since it allows me to create (once again!) groups with identical names in different places of the group hierarchy, and assignment to one such group adds the same entry to all such groups, which might be very useful (detailed discussion in https://discourse.jabref.org/t/hierarchical-groups-with-duplicated-names-are-actually-working-in-jabref-5-x/2619 ).

This is how it looks after some editing of the database:

The resulting bibtex file is added in 'experimets-with-group-hierarchy.zip'.
experimets-with-group-hierarchy.zip

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