Duffel bags are considered to be a personal item on the plane as long as your duffel bag is small enough to fit within the airlines guidelines of personal items.
Most small duffel bags can be brought on the plane as a personal item and not need to be put in your checked luggage.
If the duffel bag is larger then it will usually need to go into the checked luggage and not into the plane with you as a personal item.
I've taken small duffel bags on the plane and they counted it as a personal item but the larger duffel bags were not and had to go through checked luggage where I put my other luggage.
But if the duffel bag is small then it should be counted as a personal item and you should be able to bring the duffel bag on the plane with you.