1) In the ennies lobby arc it was stated that nobody can have more then one devil or they would die. So how come blackbeard can do it?
1.) Marco said his body structure is strange; people interpret this as he has a dead/parasitic twin (see: Kuato, Total Recall) inside him and therefore has the potential of consuming to DFs. Or he can merely eat two DFs; though this is logical, it doesn't seem to be true because Oda would never make the answer of one of his most discussed questions so generic and simple - something which can be backed up by the fact that Oda rarely goes the "simple" and "ordinary" route.
2.) Prior to eating the YYnM, he was already a DF user, the user of the Inu Inu no Mi, Model: Cerberus. This Mythical Zoan fruit allows him to consume two other DFs due to the two extra heads he possesses. This is supported by the fact he used Wolverine-claws prior to consuming the YYnM; and more heavily supported by the fact that his Jolly Roger has three skulls.
3.) The YYnM allows one to absorb the essence of a DF; so either BB made WB's DF respawn and absorbed it from the new YYnM, or he sucked it out of WB's corpse.
4.) God knows.
2) If someone dies , how is their devil fruit reborn? This was the case with ace's flame flame fruit.
The essence of the DF is respawned into another nearby fruit of the same kind; instead of growing from a plant, the ability regenerates into another existing fruit, thus the DF is reborn.
