The duplication error, that makes sure combinations of label and description are unique per language, fires off in an unwanted scenario.

Please follow the use case:

1. I am working with two items about rivers with the same name (Luncșoara): https://www.wikidata.org/w/index.php?title=Q6704038&oldid=712569951 and https://www.wikidata.org/w/index.php?title=Q4685855&oldid=708385509
2. I need to fix the title of Q6704038 (ş->ș) and also disambiguate the items by setting detailed description. Note that the initial description for both is the same: 'river in Romania'
3. I go to Q4685855, I leave the label as 'Luncșoara River' and set description to 'river in Romania, tributary of Hălmăgel', I save and it works fine.
4. I go to Q6704038, I set the label to 'Luncșoara River' and set description to 'river in Romania, tributary of Valea Mare'

OBSERVED:
On step 4, when I try to publish/save, I get the following error:

Could not save due to an error.

Item Q4685855 already has label "Luncșoara River" associated with language code en, using the same description text.

Please note that the error is erroneous, as item Q4685855 had been set with a distinct description before I altered Q6704038.

My workaround is purging Q6704038 and trying again.

Please consider reproducing using the following items: Q6711603 and Q13580581, where the latter is to be used for step 3 and the former for step 4.