Your question is Detect Duplicate Rows Without Keys. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
How would you find duplicate rows between two tables (one with about 2M rows and another with about 10 duplicate rows), and how would you find 10 duplicates within 1M records when there is no primary key?