Recently a user created a property but forget to close the property proposal. I noticed it only when the error message was displayed after I clicked the "create" button.
Also Special:NewItem says "Make sure the item does not already exist" but there're no convenient way to find them.
A possible implementation is like https://phab.wmfusercontent.org/file/data/e6gaxtomjn4swlgsadjm/PHID-FILE-dk4ar36rexnfy5lwr6gu/Screenshot_2014-10-29_13.10.48.png, though I have no idea about we should implementant it in Wikibase or as a gadget.
Note this is different from T149616: Prevent duplicate properties and items from getting created.