If multiple party members have a Gem of Efficacious Toxin at rank 25+, will the "All enemies you poison take 10% increased damage from all sources" debuff stack? So when multiple people apply the DOT, do you see 10% damage increase or more?
My research indicates that you will see only a 10% damage increase.
According to Wikia, a mob can only have one of this gem's '10% increased damage taken' debuffs at a time, irrespective of how many players are applying it:
At rank 25, it also increases damage the poisoned target takes from all sources (including other players), making this gem useful to classes focusing on elements other than Poison. If [t]his debuff is applied by more than one player, it will not stack.
This would appear to be backed up by a discussion on the US Battle.net forums:
are you referring to if all four in party uses the gem and have it at least 25%? then no, the 10% doesn't stack. mobs won't take 40%~ additional damage.