John 1:21-23

And they asked him, “What then? Are you Elijah?” He said, “I am not.” “Are you the Prophet?” And he answered, “No.” So they said to him, “Who are you? We need to give an answer to those who sent us. What do you say about yourself?” He said, “I am the voice of one crying out in the wilderness, ‘Make straight the way of the Lord,’ as the prophet Isaiah said.” (John 1:21-23, ESV)

John the Baptist, already denying that he is the Christ in the last two verses, is now denying that he is Elijah or the Prophet. It had been prophesied by Malachi (Malachi 4:5) that Elijah would return.  There were also prophecies from Moses (Deuteronomy 18:15) concerning The Prophet.  The way John the Baptist was preaching, it seemed like he might be one of these two individuals.

When Jesus was asked who John the Baptist was, his response was,

and if you are willing to accept it, he is Elijah who is to come.
(Matthew 11:14, ESV)

Is this a contradiction?  No.  When they were questioning John directly, they were asking him if he actually was Elijah.  They were wanting to know if Elijah had returned, in a literal sense. Jesus tells us that he is fulfilling the prophecy concerning Elijah, even though he is not literally Elijah in the flesh.

What might be more important is what John does consider himself to be.  It is something that we should all consider ourselves to be, in some measure.  We are simply voices, crying in the wilderness.  It isn't about us... it is all about Him.