Most Serbs who lived in Croatia could get Croatian citizenship, Serbs that were expelled from Croatia in Oluja action (1995) also can get citizenship. Many Serbs born there or descending from there now take that opportunity to obtain EU passport, since Croatia became EU member 3 years ago. I don’t know if it was automatic. Short answer yes, people who lived in one of the federal republics (SFR Yugoslavia had six of those, Serbia, Croatia, B&H…) got that citizenship provided that they wanted it, which wasn’t always the case in 1991-1995. Many can get dual citizenship, like many Serbs have both Bosnian and Serbian or Croats have both Bosnian and Croatian.