What exactly is "nostrification" of a university diploma?
Nostrification is verification that Degree X you received in Country A has the same value as Degree Y received in Country B. You do not receive a new diploma for Degree Y in Country B.

Well, actually a university in Country B may decide to give you Degree Y based on your work in Country A, but it's certainly not standard, and usually bond to university-level agreement and cooperated studies, but even then it's often not the degree that's nostrificated, but rather the thesis, its reports etc. and the defense that's considered.

The most common reason for diploma nostrification is (1) job requirements in Country B, often in governmental jobs such as offices, schools etc.; or (2) continuation of study, for instance nostrification of Masters for the admission to PhD studies in Country B. In neither case, it is necessary to have Degree Y in Country B.