While the problem of totally identical references is more serious (see T224333), this should be taken into account too: if a user tries to add a reference which is already present, although with different retrieval date (https://www.wikidata.org/wiki/Property:P813), he should at least visualize some alert message discouraging him from publishing; if a bot (or QuickStatements) tries to do it, it should be prevented from doing such edits. Otherwise, we obtain cases like https://www.wikidata.org/w/index.php?title=Q94586689&oldid=1324224486#P569 and https://www.wikidata.org/w/index.php?title=Q94586689&oldid=1324224486#P19.