Summary: Slime blocks attached to sticky pistons STILL duplicate randomly. It appears to happen when the player moves 50 - 100 blocks from the piston in question. This is currently easily reproducible.

Steps to reproduce;

1. Build simple hopper timer, using redstone torch to actuate sticky piston.
2. Start the timer by placing blocks into the hoppers (I used 5 blocks).
3. Move about 100 blocks away, then return, and observe duplicated slime block.

This can be reproduced with a new world, and the above test, please see the screenshots attached. Please note this bug is NOT a duplicate - I've reported this bug before, and it was marked as a duplicate, then marked resolved.

It is STILL an active bug in the 1.9 released today. Please check it.