This bug occurs when you place a detector rail in South - North orientation, put a hopper above and place the Hopper cart on the rail, the hopper-on-wheels will randomly suck one or two items at the same time. This is more obvious when you let the cart to drive by. It will suck two items at once instead of just one, while the hopper is powered by the detector rail underneath it. East - West orientation doesn't seem to be affected by this bug. The world file is attached.