The triplets/quadruplets theory was because that was the only scientific explanation of the existence of three corpses with the same DNA. But as established later, even identical siblings cannot share the same finger prints/toe prints/ear prints. Yet the later two corpses and the current living guy all have the same such traits, they cannot just be brothers, but some super natural phenomenon. Seems like he pretty much respawns looking thirties whenever he was killed.
The fact that he creepily knows information/events normal people, and that he calms his followers also suggest that he can do more than just respawning.
I would have loved for this to be a pure scientific mystery series, but it seems the guy will be one supernatural element, after all. He has been a fairly good one though, so all good.