I just created an item with a sitelink. When I tried to add statements to the item, I got an error saying that the sitelink was already in use by another item (not that I was trying to change any sitelinks). It seems that when I tried to create a new itemWhen creating Items and Properties on Special:NewItem and Special:NewProperty respectively, two items with the same sitelink were createdit's possible to submit the form twice.
The items are https://www.wikidata.org/wiki/Q27067644 and https://www.wikidata.org/wiki/Q27067645Submitting the form twice will lead to two entities being created or the entity being created, but the user still gets an error message (because it can't be created twice).
I started with https://www.wikidata.org/wiki/Special:NewItem?site=hakwiki&page=%E5%88%86%E9%A1%9E%3A%E9%81%93%E6%95%99&lang=hak&label=%E5%88%86%E9%A1%9E%3A%E9%81%93%E6%95%99
As far as I remembern order to fix this, I clicked in one of the input fields and pressed enter to submit the form.
I've come across items with the same sitelink created by other people before as wellthe form on the above mentioned special pages should be disabled using JavaScript when initially submitted, so that it can only ever be submitted once.