When Helga says: "Oh Arnold, what is this thing called love?" you can clearly see in the background a cloud resembling Arnold's head shape.
The title of the horror movie "Evil Twin II" is a reference to the Evil Dead series. However, the movie itself appears to be a slasher film with a chainsaw wielding serial killer akin to The Texas Chainsaw Massacre or Motel Hell.
In this episode, we learn the Sunset Arms house rules:
No children (Arnold's obviously an exception).
Craig has said in an interview that it's somewhat that Arnold is the adult of the boarding house, while the rest of the boarders are like the children.
Stinky tells Arnold "Your no Dapper Dan". Dapper Dan hair pomade was used as a fictional hair pomade for the movie Oh Brother, Where Art Thou? which released 14 months and 13 days after this episode.
In this episode, we actually see what Helga's toenails look like up closely, as up until then, the animators never drew nails on their toes or fingers (if one counts as when we see Oskar clip his big toenail in Arnold as Cupid, or when we see Helga’s fingernails as she glued fake nails in Helga's Makeover).
Arnold tells someone (in this case, Stinky) to "Shut up" for the first time.
In "Weird Cousin", when Lila and Arnie are in the cinema, we can see that they watched Evil Twin 2. In "Arnold Visits Arnie", Lulu, Arnie, Arnold and Hilda watch Evil Twin 3.
In "Weird Cousin", the love rectangle goes Arnold --> Lila --> Arnie --> Helga. In "Arnold Visits Arnie", the love rectangle is reversed, with Helga and Lila respectively replaced by Hilda and Lulu.