At first I thought that the Black Star could hold more than one soul because I could recharge two items with it. However, I have two of them in my inventory. Did this happen to anyone else?

Is one the broken one that you start the quest with?

Also, there is the version that you get at the end that will trap human souls along with any other souls, and then the one that will only trap non human souls. this depends on who you take the gem to in the process of the quest.

So, the next question is, which of the 3 possible ones are the two that you have?

Both of them are the black soul version. Nelacar gave me two, apparently.

id like to dupe some daedra hearts.

If what you say is true, then it's possible that you inadvertently and unknowingly stumbled upon a yet-unknown duplication glitch. I have previously wondered whether such a glitch were out there, waiting to be discovered.

I have had this happen with other daedric artifacts when i get them i get 2 don't know why it duplicated it but it did be happy you got that one duped.

I also have had duplication happen, though with the Staff of Magnus. Was fun completing the CoW quest dual wielding 2x Staff of Magnus-es