Ford focus roll back issue?

I took my focus into the dealership (its an automatic) because of dangerous roll back on hills (between 6-10 feet). I was told that it is Normal for the car to roll back and they sent me away. I have had the car a year and have had several instances where the car nearly rolls into the car behind me. Would I be at fault in an accident? Could I sue ford if im found to be at fault? Im really worried about rolling into another car.
Added (1). the car rolls back in drive.
Added (2). the car rolls back usually when i am at a stop and begin to accelerate. I give the car gas but the transmission wont engage. I usually end up peeling out when the transmission finally grabs. I have had 5 automatic cars and ive never had this happen before. Even modern standard transmissions are made with an anti roll back feature.
Added (3). To all you people suggesting holding my brake with one foot and the gas with another, my grandmother actually failed the drivers test because they said you cant drive an automatic with two feet. And as i said, even manual cars dont roll back anymore.

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