Ok we know Al was buying his meat for the diner in the 60's. If the meat Al has taken back to 2016 is eaten by a diner and Al goes back through the Rabbit Hole does the meat that has been eaten dissappear thus rendering the diner hungry again?
Nope, it does not disappear, it's like in quantum entanglement.... Things can actually be in two places at the same time, only if they are not being measured at the time...
This is better explained in the book,but things brought to the future remain in the future.
However, here is another on for you. Everytime they return to the past its always on the same day at the same time, so Al would go to the same butcher at the same store everytime, so he was effectively serving his customers the same Hamburger over and over again.