Riftshards Having a second Riftshards boosts the % chance to proc a Critical from 20% to 36%. Critical damage is not influenced. If the two Riftshards are different levels, the highest damage critical takes precedent.
Brutalizer The chance to proc a Bash with 2 Brutalizers is 43.75%. The Brutalizers share a cooldown. Brutalizer also stacks for higher proc % with Pestilence's innate Bash.
Grave Locket It is possible to have multiple charged Grave Lockets, but each one must be charged individually by leaving the already charged one(s) in the stash or on the ground.
Mock of Brilliance Multiple Mocks of Brilliance do not stack, either per distinct enemy hero or illusions, such as Sandwraith ultimate.