The Gem of Efficacious Toxin has the following attribute:
Poison all enemies hit for 2000% weapon damage over 10 seconds.
Will an enemy be poisoned by 4000% if hit twice?
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Instead, subsequent attacks just refresh the debuff. However, debuffs from multiple players will stack with each other. It has also been shown that the 10 % damage buff stacks when used by several players.
Another small thing to note is that the debuff is applied by pet attacks, but it is not refreshed continously by dots.