This sounds like the issue from everquest 1 /duel feature... if you die by DOTs/Dmg shield you actually died... seems the allocation of dmg shield and dots are not related to the characters dmg and is on a seperat table that when doing dmg to you doesnt register as a person doing dmg to you but a spell thus you die and the server thinks you actually did die...
Seems SOE hasnt gotten around to fixing this yet... but it does make since why you die by dot... on the otehr hand DDs should not effect duel since its instant and seems to allocat that it was the player donig the dmg