Duncan had no problem flirting with Heather during his relationship with Courtney, but completely stopped when he dated Gwen
I'm not going to argue your point, as you'll probably go insane and call me psychotic, but I want to point out when he was dating Courtney is season 1 he still thought Heather was hot, but in seson 3 when he was dating Gwen, Heather had shown her true colours and wasn't so hot, so he stopped. Much like he stopped with Courtney when she went insane with him.
Haha. No, I won't be going insane over a cartoon ship nor am I trying to start ship wars as I am merely stating my observations and opinions.
If it really is that offensive, I'm not calling every Duncney fan who disagrees as psychotic so long as they don't try shoving the "superiority" of their ship down my throat. This obviously is from my experience when sharing my views around the many Duncney fans I've interacted with mixed with a bit of exaggeration of course (Though not as much as you think). Hence, why I stated "all" and "psychotic".
Anyways, Heather essentially showed her true colours the day she betrayed Lindsay, making people dislike her. When she was snuggling up with Duncan, this occurred in the later episodes of the season while he was in a relationship with Courtney.