Has someone ate the gum-gum fruit before Luffy? I know Shanks found it but Devil Fruit do reappear after the person who ate it dies. So is it known if anyone ate the Gum-Gum Fruit before Luffy?
Shanks knew about the fruit from the "Devil fruit encyclopedia" (悪魔の実図鑑).
Oda answered this in the SBS Volume 45:


  D: Hey, Odacchi! I got a question! About Devil Fruits!! If Kaku and
  Kalifa said they wouldn't know what their Devil Fruit were until they
  ate it, how come Shanks knew what Luffy's Gomu Gomu no Mi was before
  he ate it?? Tell me! Tell me now!! Pleeeease tell me!!! P.N.
  Shirogitsunekko
  
  O: Well, I think I'll eventually get to explaining more about Devil
  Fruits in the main story, but you see, there's a book of Devil Fruits.
  It has info about all kinds of fruit names and abilities, but few of
  the fruit can actually be identified by their shape. In the case of
  the Gomu Gomu no Mi, there was a picture and everything, but for Kaku
  and Kalifa, they were talking about how they wouldn't know until they
  had finally eaten the fruit. That was in Volume 40.


Since it was in the book, we can conclude that someone must have already eaten the fruit before.