In the Zoan family of Devil Fruits, there are different "models" of similiar animal species. Chaka ate the Inu Inu Fruit Model Jackal making him "the Jackal" while Lasoo ate the Inu Inu Fruit Model Daschund. As far as I know those are the only two Fruits of the same name/different models, however all the Zoan types have a model. I think this gives Oda the freedom to keep making animal characters without worrying about his own rules and having to keep renaming the fruits.Originally Posted by Skrymir
And I assume nothing about the origins or rules of the Fruits other than what Oda has revealed, so it's my understanding that there exist only one of each.
I want to believe that Roger never ate a fruit and had something else about him that made him great. I don't believe that raw strength or super natural abilities is what makes you the best, especially in the One Piece world.
A lot of people are waiting to see what Shanks special power is or how unbelievably strong or skilled he is. Until that day comes, I hope Shanks true power is his personality, his charisma and his sense of "right". I think the government fears him so much because he has the ability to unite the people and change the world, for the better. I think these are the same qualities Roger had. He was executed because of his ideals and possibly for the information he possessed about the "true history".
I've seen people use Bounties as the equivalent of power levels, similiar to Dragonball. Like my previous examples I didn't think that was the case and I'm very pleased to see that Oda has backed this up recently with Robin. Yes she's "powerful", but her true value was based on her knowledge.