This bug is very strange and only occurs on a server. The easiest way to replicate this is to create a superflat (I used Redstone Ready) in single player, tp to X50 Y56 Z60, look to the east and type this command

/tp @a 50 56 60
/setblock 60 59 61 minecraft:piston 12 destroy

One extended piston is placed correctly. But if you use the same world in multiplayer, the piston duplicates oddly. This only occurs with extended piston and sticky piston blocks (8,9,10,11,12,13) that I can find,

My best guess is that in multiplayer, the extended pistons Y coordinate gets merged with the X coordinate and creates a duplicate. In general, it is likely not noticed because your X and Y coordinates need to be similar together to see the issue. And you can see how it is affected by lowering the Y coordinate, the duplicate moves to the X coordinate as well.