Dick York does not appear as Darrin in Mrs. Stephens, Where Are You? (1969) and his absence is explained as being in Boston on business.
The dual role of identical cousins or siblings, with contrasting personalities (Samantha / Serena), was a common plot device and special effects' technique (using split-screen) in 1960s sitcoms. It was used for twin brothers Herman / Charlie Munster in Knock Wood, Here Comes Charlie (1964), cousins Carlos / Luis Ramirez in The Convent Gets the Business (1969) and Jeannie / evil fraternal twin sister (also named Jeannie) in I Dream of Jeannie (1965).
Serena's beauty mark (left cheek just outside of the corner of her eye) represents the planet Venus and consists of a circle with a small cross below it. It originated in Late Antiquity as an astrological symbol (associated with the goddess Venus). It also represents the gender symbol for a female organism or woman.
The script uses the word "tosspot" twice, a word that has almost become archaic in the decades since then. It's a slang word meaning drunkard.