OSM shouldn't be edited to fix how Kartographer renders.

It seems like either Q23287 actually matches to two objects in OSM (which would be pretty rare) or that the Wikidata tag should only be on one, in which case it's best to actually explain that in your edit. 

I'm not from the UK so I don't know the nuances of ceremonial counties and which one would be valid, but if it's valid to be on two relations, then it should be on both and Kartographer needs to figure out how to handle that.