Numbers 23:19
God is not man, that he should lie,
or a son of man, that he should change his mind....
Is this merely repetition saying "God ain't a human being" twice in a row for emphasis, or is there more nuance?
It is certainly a poetic or literary form common in Hebrew to parallel two similar things for emphasis or greater clarity. This same parallel exists in other places--here are a few of them:
How much less man, that is a worm? and the son of man, which is a worm? (Job 25:6, KJV)
What is man, that thou art mindful of him? and the son of man, that thou visitest him? (Psalm 8:4, KJV)
Blessed is the man that doeth this, and the son of man that layeth hold on it; that keepeth the sabbath from polluting it, and keepeth his hand from doing any evil. (Isaiah 56:2, KJV)
And Hazor shall be a dwelling for dragons, and a desolation for ever: there shall no man abide there, nor any son of man dwell in it. (Jeremiah 49:33, KJV)
However, both of these particular meanings are highly significant elsewhere in Scripture.
Consider:
God is not a man, that he should lie; neither the son of man, that he should repent: hath he said, and shall he not do it? or hath he spoken, and shall he not make it good? (Numbers 23:19, KJV)
And also the Strength of Israel will not lie nor repent: for he is not a man, that he should repent. (1 Samuel 15:29, KJV)
VERSUS
When Jesus came into the coasts of Caesarea Philippi, he asked his disciples, saying, Whom do men say that I the Son of man am? (Matthew 16:13, KJV)
Before we can understand who Jesus is, it is important to understand who God isn't. For God to bring Himself down to our level is beyond comprehension. While He could not be a man like one of us, He chose to enter a man and speak through him to teach us the way to heaven.
Only through Jesus do we learn what we must know about God, who is not a man, nor the son of man.
This would be an example of what scholars refer to as "synonymous parallelism." It is a literary device, sometimes used in Hebrew poetry, where the same idea is repeated in two different ways for emphasis/effect.1
Another example of this is Psalm 120:2:
"Save me, O Lord, from lying lips and from deceitful tongues."
As such, I believe your interpretation is correct, that it's emphasizing the gap between God and man.
1Tremper Longman III, How to Read the Psalms (Downers Grove, IL; Nottingham, England: IVP Academic; Inter-Varsity Press, 1988), 99.
in Exodus 15:3 it state that he is a Man;
The Lord is a man (אִ֔ישׁ) of war: the Lord is his name. Exodus 15:3 KJV
אִ֔ישׁ = man, H376 So the focus is not on Man (Ish) nor on the son of man/mankind (Ben Adam) but the attributes of being a man.
God is not a man, that he should lie; neither the son of man, that he should repent: hath he said, and shall he not do it? or hath he spoken, and shall he not make it good?
In Numbers 23:19 what is rather being conveyed is that God is not a liar (it's impossible for God to lie Hebrews 6:18) nor won't repent of his promisses as son of men do.
In Greek LXX this distinction makes sense together with the verbs used in that sentence.
Balak has asked Balaam to curse two times in two different places. He hoped that after the first time god told Balaam to bless and not to curse, the second time Balaam may get god to be favorable to a curse addressing the god from a different place. And this is the issue that god takes in 23:19 saying that he is not disparate as men nor he can be forced to do something like children. LXX