I've seen this popup on these forums twice now and its
completely wrong information.
They fixed this issue with a patch back in 2011 when Runic Corruption was originally introduced.
Let me quote
http://wow.gamepedia.com/Runic_Corruption.
Now Runic Corruption isn't a fixed 3 second duration anymore because of how the rune system has changed over time.
But the effect is still the same.
Extra Runic Corruption procs
add extra duration to the buff. So you do get the full benefit regardless of how many Deathcoils and Deathstrikes(higher proc chance but less efficient if you want more procs) you spam.
Now there is a situation where you
do wantto avoid procing Runic Corruption.
That is when your runes are full or approaching full so the extra regen speed has no effect.
Heres an example:
You just used Festering Strike, so you have 2 runes regenerating and 4 runes ready to use, Sudden Doom procs.
Deathcoil would be the
wrong thing to do in this situation.
This is because the potential Runic Corruption proc would only hasten the regeneration of 2 runes, instead of the max 3 runes regenerating at once the Legion rune system allows.