Just had a weird bug where because I was running the Spellwarden pvp talent, Painsmith would throw double hammers and blow up the tank/raid. The spikes afterward were also doubled up and moved faster - this was in Heroic.
Soon as I swapped out Spellwarden it stopped.
So, just a heads up if you’re tanking SoD make sure you don’t have the Spellwarden pvp talent selected.
See: https://eu.forums.blizzard.com/en/wow/t/painsmith-raznal-double-casting-instrument-of-pain/297586/5