Rather than ask the question "what steps should I take?" I like to ask "what steps can I take?" -- leaving the possibility open that there will be more than one approach. I want to use multiple approaches, because experience has taught me that I will find more records that way.
- Pretend you are doing an all-new search. Don't worry too much about what you might have done in the first place -- concentrate on what you can do now. "What sources can I find that would support this assertion?"
- Pretend you are working a same-name problem. You have your main person, and this other person who was born in Yugoslavia. Try to find records to show these are two different people. (Let the evidence itself drive you back to the conclusion that all these records refer to the same person.)
- Pull together all the sources you have which mention that individual, and write a fresh biographical profile and research plan. Branch out to include siblings and the FAN group/clusterFAN group/cluster (Friends/Associates/Neighbors) as needed.
- Use the assertion as a clue to what source it might be. What if the person was asked for their year of birth, and during the date the record was made, the administrative name of the place where they were born would be described as "Yugoslavia"? What time period would that be? Which informants might use that name?