The specific rule is that a personal item must be able to fit under the seat in front of you, in that space, without any sort of extension into the aisle.

At most gates, I've seen the little boxes that are used to size test carry on luggage to also have a section to size test personal items.

My experience is that it barely fits a large backpack with some books, snacks, and at least one 15 inch laptop.

Carry on luggage can be much, much larger.

> I frequently travel with a duffel bag carry-on that goes in the overhead

Yes, that is one carry on luggage item, and one personal item.

>They could also just say "only one carry-on can go in the overhead bin."

No because you are not afforded two carry on luggage items. One item has to be small enough to fit under the seat in front of you.