I thing it could be useful to add an Option so that a Property can only be used once on a page. Maybe something like [[Only usable once::true]] and [[Only usable once::false]].
The Idea is that this could prevent unwanted output in some inline queries. And in bigger wikis with a lot of users this could happen fast.
In the case that a Property with the Option [[Only usable once::true]] is used twice or more in one page only the first Value schould be used an there schould be shown an errormessage.