Contradiction with Matthew 11:14
This verse states that John the Baptist is Elijah, contradicting John 1:21 where John denies being Elijah.
Matthew 11:14: And if ye will receive [it], this is Elias, which was for to come.
Contradiction with Luke 1:17
This verse claims John the Baptist came in the spirit and power of Elijah, which contrasts with John 1:21 where he denies being Elijah himself.
Luke 1:17: And he shall go before him in the spirit and power of Elias, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just; to make ready a people prepared for the Lord. [to the wisdom: or, by the wisdom]
Paradox #1
John 1:21 might seem inconsistent because it shows that people were unsure about the identity of John the Baptist, asking if he was Elijah or another prophet, which contrasts with later passages where Jesus identifies John as Elijah in a symbolic sense. This can seem puzzling if taken literally.