Currently properties that link to non-file Commons pages , like Commons category (P373), Commons gallery (P935), Commons Creator page (P1472), or Commons Institution page (P1612), are saved as string type properties. As a result a link to a single page can be saved in numerous valid but different formats that point to the same page. For example Commons Creator page (P1472) can be set to "Stanisław Lem", "Stanisław_Lem" or "Stanis%C5%82aw_Lem" and they will all point to c:Creator:Stanisław Lem. Wikidata should accept inputs in any of those forms but save it in a uniform format. Otherwise we can have multiple links from a single item to the same page, it also makes it hard to write a query which compares such properties to sitelinks or to each other.

One solution would be to either extend commonsMedia property type to allow links to other pages on commons. Other solution would be to create property type specifically designed for linking to pages on commons and other projects.