Some users are asking how to deal with towns which are seat of local government (mainly for admin_level=8). Wikipedia generally didn't have different id for the city and the government, but in OSM there's a place=town and a administrative relation.
Initial discussions lean to add the wikidata tag to the relation.
Would it be a problem if both the node and the relation had the same Wikidata ID? I don't think so.
If possible, I would prefer to reduce information duplication as much as possible. Copies often lead to stale data and errors. If there is a relation that includes a node, it might make sense to keep just one wp/wd tag. On the other hand, Wikipedia at this point only uses "areas" and "lines" (both of which could be generated from a way or a relation), so tagging something on a node doesn't really affect it.
The node usually is added to the relation with the role admin_centre in various countries.
