Dispensers sometimes operate incorrectly

While this glitch is somewhat rare, it is bothersome:

Dispensers sometimes make TWO things (arrows and fire charges) come out of it at the same time. This is upsetting because it is using both items instead of one. This problem basically renders mob traps useless because both items are used to shoot, instead of one at a time. The dispensers are behind a pressure plate, Because of this, the items get consumed faster than they should. Nothing else is connected to the dispenser or the pressure plate.