|Kor - Orc Scrollkeeper|
RE: Sickening Critical and Stunning Critical (probably others as well)
These feats both state: "The effects of this feat do not stack. Additional hits instead add to the duration."
I'm guessing that the effects don't literally "add", but instead extends/prolongs/replaces the duration.
Sickening critical lasts 10 rounds. If you get hit again with this 2 rounds later, I'm assuming the duration is now 10 rounds, and not 18.
Perhaps some wording clarification is in order though? "The effects of this feat do not stack. Additional hits replaces the duration."