Cerberus Theory - Blackbeard Had Already Eaten a Devil Fruit
The Cerberus Theory, easily one of the most popular within the One Piece community, posits that Blackbeard had already eaten a Devil Fruit before he ate the Yami Yami no Mi. Specifically, a Mythical Zoan Fruit possibly called the Inu Inu no Mi, Model Cerberus. Theorists believe that due to the nature of the actual mythical Cerberus having three heads with independent thought, this allows Teach to use more than one Devil Fruit. The justification for this theory mainly comes from how Blackbeard and his actions tend to revolve around the number three.
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First is Blackbeard's Jolly Roger, meant to represent a ship's captain. His specific flag features the standard skull and crossbones, but also curiously has three skulls that are quite close together -- like that of a Cerberus. Then there is his supposed weapon of choice, a three-bladed claw he's used since he first joined the Whitebeard Pirates. This claw is actually responsible for Shanks' famous scars across his left eye -- or it could have been the claw of Blackbeard's hypothetical Zoan form. The fact that the myth of Cerberus exists directly in One Piece's world gives this theory even more credibility.
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Chimera Theory - There Are Multiple Entities Inside Blackbeard
Much like the Cerberus Theory, the Chimera Theory operates on the idea that there are multiple entities within Blackbeard. While the former implies they come from eating a Devil Fruit, the Chimera Theory states that this is just how Blackbeard was born. Given how Marco the Phoenix even said during Marineford that Blackbeard's body was unique, there is a bit of in-canon One Piece evidence to support this theory.
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While quite a rare occurrence, there have been multiple cases of fetuses absorbing one another while in the womb before they are born. This is commonly referred to as a "Vanishing Twin." This process essentially creates a person with multiple DNAs and sets of genetics, which can result in abnormalities remaining within the body after birth. This can even potentially give that same fetus multiple sets of organs.
It is quite possible that Blackbeard's body actually has multiple "beings" inside of him, which is why having multiple Devil Fruits doesn't kill him. If a person can only have a single Devil Fruit and Blackbeard was actually made up of multiple people, then it is entirely possible his body could utilize as many Devil Fruits as there are people within him.