Donald E. Westlake 
Two Much! 

A ruse to bed a pair of twins quickly grows complicated Art doesn’t mean to tell Liz Kerwin that he has a twin. He’s on Fire Island, and she’s so beautiful that he’s willing to say anything for a chance at getting rid of her clothes. So when Liz mentions an identical twin sister, Art blurts out that he has a twin too. His name is Bart, he says, and describes the most boring man he can dream up. Liz thinks he would be perfect for her sister Betty. When Art meets Betty—who is, of course, just as lovely as her twin—she asks about his brother. Hoping for a chance at the family fortune, Art dons a pair of glasses, slicks back his hair, and soon has “Bart” engaged to the sister. As his simple lie spins out of control, Art learns that wooing sisters is never as easy as it seems.