And I think I know how I got them.
For the consistent readers, I'll back up a bit. Ky the Dark Elf is not going to be played much more, I made a Nord warrior type to see how two handed weapons were. While they were fun, I disliked the idea of ruling out any stealth in the future. So, I rerolled as an Orc. The results so far are extremly satisfactory.
The first objective I had was to get a house in Whiterun. Did that soon after becoming Thane.
Second objective was to take advantage of the Ancient Knowledge bonus by wearing Dwarven Armor. Check that box.
Third was to collect items to craft Daedric equipment. To that end I did the Azura quest at an early level and got the four hearts from thos Daedra. Then, I did the Mehrunes Dagon quest and got not only the Razor, but also the four respawning Dremora. I've cleared them twice. Finally, I went to Gloombound Mine and harvested all the Ebony
Fourth in line was to get 100 in Smithing. To that end, I mined all the Iron availible and made Iron Daggers. That still only got me so far, so I decided to attack that in a roundabout manner. I worked my way to the Harbinger of the Companions and repeated Animal Extermination quests from Aela to get Pelts to turn into Hide Bracers. That finished me off on that objective.
In between objectives three and four I did a bit of side questing to get a neat little Dagger called Bloodthorn. Bloodthorn only has enough charge for about fifteen strikes, but it has a small Absorb Health enchantment that makes me nearly invulnerable. On top of that, it soul traps my targets. When I take that with the Black Star, I have unlimited energy for the thing.
Now on to the subject of my title. Three. So, I pick up the Bloodthorn and I decide to test it out on those silly Bandits hanging around Fort Greymoor. During the epic slaughter, I notice that the 'A Soul has been Captured!' notification displayed twice after one kill. So, I go to empty the most recent soul into my Dagger and I see two Black Stars in the list of soul gems I can use. Two. Black. Stars. I empty one of them, then I look down my inventory list to Misc. to see if there were. There was one with a Grand Soul, and one that was empty. I laughed in disbelief and finished clearing out the fort.
Later, the same sort of thing happened. Double message, duplicated Stars. Both instances occured when I was using the Bloodthorn, and when I was fighting Humans, or at least Elves. Most importantly (I think) both occured when I killed the target with a power attack.
I now enter into speculation, because beyond what I just wrote, I have no idea what happened. I've noticed that power attacking with Bloodthorn results in a double tap of my dagger on the target. These two attacks come in tightly grouped. I think that that double tap made the Dagger hit the target and kill it, but then hit it again before the target was registered dead. Since the enchantment is on the weapon, it might soul trap the target twice or something.
I have also noticed (upon trying to dupe the Star some more) that Bloodthorn duplicates Black Soul Gems and Grand Soul Gems, I have not been keeping track of gems that are lesser than those but I suspect the same thing is true. I have also noticed that sometimes when the double message of capture is shown that two Black Stars are filled. I've found this exploit on my 360, I don't know if it works on the PS or PC.
Finally, the behavior of these Stars is very odd. One of them seems to operate on the rules explained by Nelacar, that only Black Souls can enter it. One of them seems to think that it is Azura's Star, and it only allows White Souls of Beasts to use it. The other will hold Black and White Souls alike.
Finally, there are 422 Egg Sacks under the Frostflow Lighthouse. I harvested them all.
Every. Damn. One.