If i can recall, i still could pop my hamstring on a target with COC or blizzard slows or any other effects similar.
I don't really think it works like this, like how you cannot have permafrost on target if it's already affected by your cone of cold or blizzard.
"Snare debuffs do not stack. Instead, the debuff that gives the most movement speed reduction takes precedence. If any lesser snare is applied while a greater snare is active, it will not be applied, even if the greater snare will expire sooner." From Wowwiki