I tested this with other players in PvP.
If your opponent walks through your poison clouds (poison or plague javelin), they will take dmg equal to what is written on your character screen.
However, if you throw poison or plague javelin directly at your target, and that javelin connects (going through defense and block), then that opponent will take TWICE the dmg over the course of the same poison duration.
I think this phenomenon happens because the poison dmg from both skills is added to the javelin itself, and the cloud and javelin connect on your target on the same frame, so the game adds both damages together, but keeps the same duration.
I’m not exactly sure if it was like that in D2 legacy, but I asked a D2 veteran if this was the case, and he said he’s never heard of it.
That’s not how poison stacking is ‘supposed’ to work
Poison/Plague javelin have two different forms of the poison damage: Attack damage added directly to the physical projectile, and the clouds also deal it to anything in the AoE. Both sources should have the same poison bitrate (dps) and duration, so getting hit by either of them normally has the same impact.
One way it could be scaled is that when you use multiple sources of poison damage on an attack, they stack in funny ways. The DPS of all sources applied in the same attack is always added together, both items and skills. The duration is trickier, its averaged for items, but disregarded if skills are involved and set to the skill duration, and if multiple skills are involved their durations are added.
So if you had other sources of poison attack damage, like venom, or poison charms, and hit someone with a plague javelin, then it could do considerably more damage per second if the physical projectile hit (and summed the DPS from sources) than if the spell’s clouds hit (and only applied their own).
However, this doesn’t preclude the possibility that they just goofed something up with it in 2.4 and now plague javelins deal twice as much if they hit directly
Good info, ty 
The DPS of all sources applied in the same attack is always added together, both items and skills.
This is probably what’s happening: flat psn dmg added directly on the javelin (not the clouds) is twice as high by mistake.
To be clear: The DPS of all attack modifier sources are added together. Not all poisons from all sources applied in the same frame. When multiple poison sources hit in the same frame, they are all mutually exclusive, only one will apply (a higher DPS always overwrites current poison). If you get hit by the javelin and cloud in the same frame, only one applies, whichever rolled higher in the random range, but they should have the same poison values, and it should never result in double DPS.
It would only explain the DPS being higher if you had a whole ton of poison charms or venom active, or if there’s some new glitch like they accidentally changed the poison applied to javelins to be twice the rate of the clouds, for no good reason
Just to be clear, my zon didn’t have any psn dmg from charms or items.