Good question. Usually the defender AI is such a way that it will go after the troops that activate them 1st. Then the next nearest troop.
So if that original troops is not destroyed will they go after the decoys 1st?
one thing to keep in mind is that troops that are attracted to the decoy move at double speed! Also, if it's in range of TC, donated troops go straight for the decoy, ignoring any troops on the way