Well, the dragonborn has a soul of a dragon, and can absorb dragon souls when a dragon dies. It stands to reason that one dragonborn can absorb another dragon soul (and possibly any souls they may have captured in) if they are around at the time of death. It stands to reason then, that if you kill a dragonborn that didn't even know they were a dragonborn, you would absorb a soul from them.
Only one way to find out...
Go to Akavir and kill Nerevarine. Now , OP!
Or...use the console and clone yourself, then proceed to kill the clone.
Maybe you'd absorb your own soul? o_o