This was my first reaction too, but then I thought back to when I was first learning SQL and, after hours of work trying to tabulate events from users across multiple tables, discovered that I seemed to be double-counting (or more) in mysterious ways. I was using Venn diagrams to understand how joins worked and I think the "intersection" depiction of inner joins _did_ cause me to think that "duplicates would be handled," whatever that would even mean. Really it just caused me to skip over the complexity of thinking about it.

If you think of inner joins as a cross join with a filter, then we have all seen cross joins represented by Venn diagrams - and for me at least I lost several hours of work restructuring my understanding of what I was working with!