Editing an existing claim via the API wbsetclaim module will result in creating a new claim. An API call looks like:
/mediawiki/core/api.php?action=wbsetclaim&format=json&claim=%7B%22type%22%3A%22claim%22%2C%22mainsnak%22%3A%7B%22snaktype%22%3A%22value%22%2C%22property%22%3A%22p1110%22%2C%22datavalue%22%3A%7B%22type%22%3A%22wikibase-entityid%22%2C%22value%22%3A%7B%22entity-type%22%3A%22item%22%2C%22numeric-id%22%3A2647%7D%7D%7D%2C%22id%22%3A%22q2647%24574052AD-1518-4CD3-A21C-637837C7C040%22%2C%22qualifiers%22%3A%7B%7D%7D&token=%2B%5C&baserevid=17420

The API result seems to be correct:
{
"pageinfo": {

		"lastrevid": 17426

},
"claim": {

		"id": "q2647$574052AD-1518-4CD3-A21C-637837C7C040",
		"mainsnak": {
			"snaktype": "value",
			"property": "p1110",
			"datavalue": {
				"value": {
					"entity-type": "item",
					"numeric-id": 2647
				},
				"type": "wikibase-entityid"
			}
		},
		"type": "claim"

}
}

However, featuring one claim in the beginning, the entity will have two claims now which can be verified in the UI.
The operation seems to mess up the revisions since removing a claim via the UI seems to trigger the "remove" action on a wrong revision.

Version: unspecified
Severity: blocker