If you hook up a comparator clock to a sticky piston and place a block from the side at the place in which the head of the piston would be if not extended, it will either not place the block, in which case you try again, or you place the block and it creates 2 of them.
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duplicates
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Marking as Private as this is a duplication bug.
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Confirmed with pistons facing North, West and South once each. Sometimes occurs, sometimes it does not. Piston facing East fails to follow the redstone pulsar and remains permanently extended until clock is disabled.
EDIT: Attached screenshots for clarity.
EDIT: Re-opening the world will cause the duplicated block to vanish. Mining the duplicate block will make it disappear without dropping any items. However, you can stand on the block or place blocks on top/beside the block. Possibly creates a duplicate Block 36.
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22nd Duplicate of MC-631, please use the search function to see if your bug has already been submitted. Currently over 56% of tickets are being closed as duplicate.