Deadly Poison only allows one debuff stack applied in a group or raid, even if multiple rogues are using deadly poison. Meaning any other rogue applying deadly poison will not get any affect from it when attacking the same target.
In example. Three rogues in a raid have deadly poison applied to their weapon. The rogue who gets the first application applied to the mob/target/boss will be the only rogue who is able to have deadly poison on the mob/target/boss (as long as the stacks stay up). The other two rogues deadly poisons will not apply to the boss or even contribute to stacks. They just do nothing. This is not an intended feature and as long as I have played (18 years of WoW), each rogue has been able to have their own deadly poison debuff applied. Just as if several warlocks were using corruption in the same target.
Thanks and looking forward to getting this fixed.