sierra said that gwuncan liked eached other in season 1 (tdi) then why duncan was dating courtney?

I honestly didn't get this reference. Sierra is a superfan and obviously picked up on the DxG vibe that everyone got in TDA. However, my only thought would be that Duncan did have a thing with Gwen in that they had a few common interests and they actually got along very well in Season 1.
I believe that he was with Courtney in season 1 because Courtney was kind of like that forbidden fruit. The bad boy always chases after the good girl that he knows he can't have....because she won't allow it. I think of DxC as more of a game. Duncan played the game......and as Gwen told him, "he played it well." Duncan tested Courtney....and she fell for him. Granted, I think he did truly like her a lot and their relationship started off great. However, I feel that TDA just ruined the relationship. He saw Courtney's true colors and saw her in a very unflattering light. Thus, Duncan realized Gwen was the better choice for him.