10 And Jesus answered and said unto them, Elias truly shall first come, and restore all things, [ One evidence that this Elias is not John The Baptist is because it is not John the Baptist's mission to restore all things. ] as the prophets have written. [ Or those things contained in the Old Testament. ]

11 And again I say unto you that Elias has come already, [ Joseph Smith's translation on this He's saying look Elias will come and restore all things. In other words there is one Elias that will come and restore all things, and there is another Elias or someone else that has already come. So in other words "Elias" is a title. ] concerning whom it is written, Behold, I will send my messenger, and he shall prepare the way before me; [ This is a specific reference to John The Baptist here. ] and they knew him not, a [ Isaiah 40. ] and have done unto him, whatsoever they listed.

12 L ikewise shall also the son of Man suffer of them. [ Just like John The Baptist be put to death so to shall Christ be put to death. ]

13 But I say unto you, Who is Elias? Behold, this is Elias, whom I send to prepare the way before me. [ Specifically here Elias is represented as John The Baptist. He is also representing John The Baptist in a way as the spirit of Elias. Here we have John The Baptist acting as the Elias for the first coming of the Savior. So the question is - Can someone hold the keys of Elias without being Elias the person? We get the answer in D&C 110:113-116. ]

14  Then the disciples understood that he spake unto them of John the Baptist, [ So the disciples recognize that he is talking about John The Baptist here - who will act as an Elias; but it does also not confer that John The Baptist was on the mount of transfiguration. At the time of the transfiguration John The Baptist was not resurrected nor was he transfigured because he was yet dead. ] and also of another who should come and restore all things, as it is written by the prophets. [ Also another Elias - someone entirely different who will come(a future event) and restore all things as was referenced in verse 10 above. John the Baptist was a preparatory Elias before Christ comes in the flesh - his first coming. Elijah is a preparatory Elias before Christ comes at the second coming. One Elias for the first coming one and another for the second coming. Both with the spirit of Elias. John The Baptist comes with the Aaronic authority, the second comes with the Melchizedek authority. ]