Damage over time...does it stack?

I'm talking teemo's poison on hit, burning damage, etc. When it says X amount of damage over 2 seconds on hit, if you hit them again at 1 second into the effect does it stack a second 2 second length of damage on top of the first, or does it simply extend the original for another second? If teemo has high attack speed, can he have 3 or 4 poison hits draining you simultaneously before the first one expires?