Gravity-affected blocks duplicating when affected by certain piston conditions

I was recently creating a hipster door design (a piston door in which all blocks retract into the floor instead of to the sides) and discovered a duping bug that can not only be exploited, but also means that the commonly used design no longer functions. It occurs when sand, gravel, and other similar blocks are pushed rapidly by pistons and are stacked in pillars. I have not been able to find a consistent pattern for the bug as it seems to occur spontaneously. When pushed by pistons, there is a chance the affected blocks will turn into items but duplicate into two of the item. For example, if I were to create a piston door design using falling sand blocks, there is a chance that when activated, the top sand block will turn into two sand items.