EDIT

The error message currently looks like an unhandled error. The message should only inform about the fact that duplicates are not allowed and therefore the selected area could not be added. No stacktrace should be shown (and also no stacktrace should be created! in any logfile as this should be a handled case).
Also only 1 message should be shown if >1 terms are tried to be added which already exist. this was partly fixed and information about the doubled terms are added.

copied from #8477: do not allow editing the flag if duplicates exist in the collection

when changing the title of the tree the part which handles the details view area which holds the 3 flags (allow duplicates, is flat and order relevant) becomes orange. As these flags are not relevant for the titleCache this should not be the case this seems to be fixed in the meanwhile

"Paste" should not be available in context menu if the area already exists in the tree

d&d from 1 term tree to another in the same TermTreeEditor does currently not do the check (and therefore duplicates are possible), while d&d from e.g. term editor to term tree editor does check if duplicates exists (same way as described in first issue above)

d&d from a vocabulary to the term tree should not be possible if the term is a duplicate