Contradiction with Isaiah 45:5
Here, God declares there is no other God besides Him, similar to Psalms 18:31; hence, it may offer a complementary rather than a contradictory perspective.
Isaiah 45:5: I [am] the LORD, and [there is] none else, [there is] no God beside me: I girded thee, though thou hast not known me:
Contradiction with 1 Kings 8:23
Solomon declares the uniqueness of God among the heavens and earth, similarly reinforcing what is expressed in Psalms 18:31.
1 Kings 8:23: And he said, LORD God of Israel, [there is] no God like thee, in heaven above, or on earth beneath, who keepest covenant and mercy with thy servants that walk before thee with all their heart:
Contradiction with Isaiah 44:6
God affirms His identity as the only God, like Psalms 18:31, thus not presenting a contradiction but rather a reinforcement.
Isaiah 44:6: Thus saith the LORD the King of Israel, and his redeemer the LORD of hosts; I [am] the first, and I [am] the last; and beside me [there is] no God.
Contradiction with Deuteronomy 4:35
This verse explicitly states there is none else besides God, echoing Psalms 18:31 rather than contradicting it.
Deuteronomy 4:35: Unto thee it was shewed, that thou mightest know that the LORD he [is] God; [there is] none else beside him.