Yeah I've been wondering that myself......I'm wondering if it is only possible to have one of each type of fruit! Blackbeard has one Paramecia and one Logia!
So Maybee that is the exception when eating devil fruits. The problem about trying to get multiple ones is that you don't know which fruit is which!!! Since most fruits are Paramecia, then Zoan (Logia being very rare), it makes sense most people die because they get a second Paramecia!
Until something like that actually happens....I'm guessing that it either is taken away or, eaten by another straw hat, or is given to Ussop's Kabuto!
EDIT: Forgot about Blackbeard being called as unique in body.....Maybee only Will of D people csn do so....I'm curious to see what happens