en_tn_condensed/jhn/08/18.md

I am he who bears witness about myself

Jesus bears witness about himself. Alternate translation: "I give evidence to you about myself"

the Father who sent me bears witness about me

The Father also bears witness about Jesus. You could make it explicit that this means Jesus' testimony is true. Alternate translation: "my Father who sent me also brings evidence about me. So you should believe that what we tell you is true" (See: rc://en/ta/man/translate/figs-explicit)

the Father

This is an important title for God. If your language must state whose Father this is, you could say "my Father" since Jesus switches to that in the following verses. (See: rc://en/ta/man/translate/guidelines-sonofgodprinciples)