Welcome to your interview.
The question is on your right: Find Duplicates in One Pass. Take a moment with it first.
Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.
Given an array of integers, how would you find the duplicate elements in a single pass?