I think in the end we develop our own way of talking at even a personal level. I say the half part, and the half but others might consider this snobby or dialect or something else wrong...
Duolingo does its best really. Nothing is entirely correct in life. Ha det på badet.
I have THE whole loaf of bread = Jeg har hele brødet.
I have A whole loaf of bread = ?
I have two whole loaves of bread = Jeg har to hele brød.
I have THE two whole loaves of bread = ?