As far as I know, after the wars in ex-Yugoslavia it was forbidden to hold double citizenship (of two ex-Yugoslav countries). Today I am pretty sure many people hold double citizenship, but to answer your question you would hold citizenship of the country you live in. If you live abroad you can hold citizenship of the country that your parents would be born on today’s map, though I think they too need to have citizenship of their countries of origin.
Serbs from Croatia had refugee status for long period, that’s until some of them returned back to Croatia, but I think most of them still lives in Serbia