Can duplicate blocks with fast firing piston

relates to:

https://mojang.atlassian.net/browse/MC-1677

https://mojang.atlassian.net/browse/MC-5726

if a block is placed on the face of a horizontal, fast firing piston during a small window where the piston is retracted (aim for bottom edge), the block can be duplicated.
(not seeing any open copies of this bug, it seems to be a repeat offender though)