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Sure, basically any time you have a many-to-many relationship you’ll have to repeat keys multiple times. Think students taking courses. You’d have a students table and a courses table, but the relationship is many students take many courses. So you’d want a third table for lookups where each row is [student_id, course_id].
This stackoverflow post has a similar example with authors and books - https://stackoverflow.com/questions/13970628/how-do-i-model-a-many-to-many-relation-in-sql-server#13970688
Thanks, OP seemed more curious about the technical aspects than just the absurdity of the comment (since pretty much every business uses SQL) so hoped a more technical explanation might be appreciated.