The only problem with this sort of identical birthdays is that they either are a match, or someone had the date and someone else with in incomplete or nonexistent date and assumed they were the same. So where exactly did the two birthdays come from? if both are from official documents, you may be fine. But if one of them is from, say, an unofficial source you have to be careful. When I'm working of a family, and come across such situations I put a note on the person / profile saying, "I am assuming thus and so. I will change things if needed.