Contradiction with John 5:31
Jesus states that if He bears witness of Himself, His witness is not true, contradicting the premise of accepting witness in 1 John 5:9.
John 5:31: If I bear witness of myself, my witness is not true.
Contradiction with John 8:14
Jesus says His witness is true even if He bears witness of Himself, contradicting the acceptance of witness needing to be of God.
John 8:14: Jesus answered and said unto them, Though I bear record of myself, [yet] my record is true: for I know whence I came, and whither I go; but ye cannot tell whence I come, and whither I go.
Contradiction with John 8:17-18
Jesus affirms the validity of His testimony and the Father's, whereas 1 John 5:9 talks about believing in human testimony over God's.
John 8:17-18: It is also written in your law, that the testimony of two men is true.
Contradiction with 2 Corinthians 13:1
Emphasizes the need for two or three witnesses for every word, contrasting with the singular testimony of God mentioned in 1 John 5:9.
2 Corinthians 13:1: This [is] the third [time] I am coming to you. In the mouth of two or three witnesses shall every word be established.
Contradiction with Matthew 18:16
Requires two or three witnesses for a testimony to be established, opposing the singular witness of God referenced in 1 John 5:9.
Matthew 18:16: But if he will not hear [thee, then] take with thee one or two more, that in the mouth of two or three witnesses every word may be established.