There are a bug of duplication of block with sticky pistons.
If you move far away from the piston, the block on this piston
will be duplicated when you be back.
Process:
put a redstone clock connected to a sticky piston
put a block on this sticky piston
run the clock
leave and be back, so that the block will be duplicate
(this bug break lot of redstone machines in 1.9)
but all changes make by this bug are fixed if you reloaded the map,
(ghost blocks and modifications) that's really strange !
I put screenshots before and after the bug effect