Hoppers outputting multiple items at once

There is a bug i stumbled across while making a keycard system. The bug makes it so that hoppers will drop two items into the hopper below instead of one. If i break and replace the hoppers above and below it fixes the problem, only to come back next time I log in. This makes the system really inconsistent, and a fix would be great before 1.10 comes out. Thanks Mojang