John's Father Is Peter's Father only son.
What is John from Peter.
Best way to do it, is to make a family tree...
Let Jerry = John's Dad.
Let Patrick = Peter's Dad.
Patrick -----> Peter
Patrick ------> Jerry
Which means they're brothers.
Patrick ---- Peter
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Jerry
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John
So to answer your question, John is Peter's nephew.
But since Patrick's only son is Jerry, that means Peter was adopted, and that makes him John's adoptive uncle. Only makes sense if Patrick only has one son, UNLESS Peter was a woman at birth, and he decided to get a sex change
which makes it more reasonable for the only ONE son description.
If John's father is Peter's father's only son, what does that make Peter? A girl? Because John's father and Peter share the same dad, so what is Peter?