Contradiction with John 5:31
John 1:19 discusses a testimony about John the Baptist, while John 5:31 implies that one should not accept self-testimony because it is not valid.
John 5:31: If I bear witness of myself, my witness is not true.
Contradiction with Matthew 11:11
John 1:19 presents John the Baptist as a significant figure, while Matthew 11:11 suggests that despite his greatness, the least in the kingdom of heaven is greater than John.
Matthew 11:11: Verily I say unto you, Among them that are born of women there hath not risen a greater than John the Baptist: notwithstanding he that is least in the kingdom of heaven is greater than he.
Contradiction with Luke 7:19-20
John 1:19 records the testimony of John the Baptist unambiguously, whereas in Luke 7:19-20, John himself sends messengers to ask Jesus if He is the one to come, showing uncertainty.
Luke 7:19-20: And John calling [unto him] two of his disciples sent [them] to Jesus, saying, Art thou he that should come? or look we for another?
Contradiction with John 1:15
John 1:19 talks about the testimony of John the Baptist, whereas John 1:15 shows John testifying about Jesus being greater, not about himself.
John 1:15: John bare witness of him, and cried, saying, This was he of whom I spake, He that cometh after me is preferred before me: for he was before me.