There is a glitch with pistons causing one to duplicate. This happens when one piston facing up is placed and another piston is placed on top of it facing and direction apart from up or down.
The picture attached is 3 screenshots showing how this is done.
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Cannot reproduce. Are you sure that you have 13w06a and not 13w05a/b?
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Duplicate of MC-8198, please use the search function to see if your bug has already been submitted. Currently over 51% of tickets are being closed as duplicate.