DoT Stacking

[Wilshire]

Do DoTs stack with others of the same spell?

For example, if two necros are duoing, will their Darkness spells stack if each necro uses it?

Same with two druids. Would their DoTs stack with each other (if each druid uses Insect Swarm, will both spells do damage)?

If I remember right, in Classic EQ, they did not, so there's literally no point in having two necros in a group or in a raid. However, that did change at some point (today in live, necro spells do stack as long as both are cast by different toons) but I can't remember when that changed.

I know that a single player can't put two identical DoTs on a mob, but can two different players use the same DoT and have it stack?