If there's only one of each fruit, how could Blackbeard recognize the "Dark Dark" fruit?

Question

If someone had seen it before you'd think it would have been eaten.

And I also thought I remember a segment about no one knowing their fruit before they ate it. (I think it was with the CP9 arc)

Answer 1

I can't recall exactly when this was said during the series, but as the WIKI says here, when the user dies, the fruit he ate simply regenerates inside the closest appropriate fruit the same place where the user died and their ability is reborn into another fruit of the same kind instead of growing from a plant.

It's only my assumption but maybe CP9 did not know what the capabilities of their new fruit were because they were not so rare.

I think Blackbeard followed the progress of the user of Dark Fruit, so he knew when and where it is.

Answer 2

As Hashirama Senju said, Devil Fruits regenerate when its user dies (it's shown for the first time during the Punk Hazard arc), so more than one person can have the same Devil Fruit ability (not at the same time, of course). In the One Piece world there are some books that give details into some Devil Fruits, so that's how Blackbeard knew just by looking at it that the fruit Thatch found was the Dark-Dark fruit, and also about its powers; that's why he wanted it so much. During the Thriller Bark arc, Sanji also mentions reading in a book about the Suke-Suke (Clear-Clear) fruit.

As for why the CP9 didn't know what the fruits Kaku and Kalifa ate were, that's because even though some fruits are known from previous users, not all of them are.

Answer 3

It has been established by now that Devil Fruits respawn. Therefore when a Devil Fruit is eaten, their shapes and powers are recorded and passed on mouth-to-mouth as mentioned at this question. Seeing humanity has been around for a while, it would make sense that the shape and powers of most fruits are recorded by now.

To come back to your question how Blackbeard in particular knew about the shape and the power of the Dark Fruit, it was mentioned in chapter 440, during his fight with Ace, that Blackbeard had memorized the shape of the Dark Fruit. He memorized its every curve and point. He must have read or heard about its powers and shape, wanting the power for himself.

He did not follow the progress of the Dark Fruit as mentioned in the answer by Hashirama Senju. Instead, he joined the Whitebeard crew and spent decades on his ship, because Blackbeard thought his chances of finding the Dark Fruit would be the best. In the end, he was correct in thinking so and despite giving up, the fruit appeared.