Prerequisites: Two items with the exact same label but different descriptions.

Change the description of one item to be the same as the other item's description. The description appears to be saved but reloading the page reveals it is not.
The API response does not return a "lastrevid" parameter. So, when changing one of the descriptions to match the other one and instantly trying to change the description again without reloading, a JavaScript error locking the page is caused since the revision id is undefined.

Probably, the API should return an error that an item with the same combination of label an description exists already or it should simply save the combination (just like it is possible to create an item with a combination of label and description that is already in use).

Version: unspecified
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=43720