Problem: Although with T100933 we already have this check for Special:NewItem, it's still possible to create an Item with the same label and description in a certain language through other interfaces, and it's always possible to set them once the Item has been created. The same should also not be possible for Properties.
GIVEN an Item or Property
WHEN adding a new label or description in one language that leads to them being identical
OR changing an existing label or description in one language that leads to them being identical
THEN an error is shown
AND the edit not saved
- Given any Item or Property, it's not possible to set the same non-empty content for the label and description in that language via UI or API