As a Wikidata editor, I want property labels to be unique per language at all times, so that different properties don’t get confused for one another (and so that the {{#statements:propertyName}} parser function is unambiguous).
As a Wikibase developer, I want hard constraints to actually be enforced as strictly as reasonably possible.

Problem:
While working on T289473, we realized that there’s a possible race condition when creating properties, very similar to T291377: if you simultaneously create two properties with the same label, it’s possible that both of them will save successfully, and you’ll have two properties with the same label, even though property labels are supposed to be unique.

Example:
Not yet tested.

Screenshots/mockups:

BDD
GIVEN
AND
WHEN
AND
THEN
AND

Acceptance criteria:

Open questions: