The purist view of Wikidata is that it’s just a collection of claims made by others. So if there are two competing ideas about the area covered by “Serbia”, however defined, then those claims can coexist with qualifiers saying who supports or rejects each claim, or in what context each claim is valid.
If the conflicting statements get at the fundamental question of what Serbia is, then ideally there would be separate items. I spend as much time conflating and deconflating items in Wikidata as I conflate and deconflate dual-tagged features in OSM. Both projects are a work in progress.