How do people on television shows get a clone of them even though they don't have a twin?

Most TV shows do this with editing, and using the exact same actor/actress. For example they shoot the same scene twice, one with Jannette playing Sam, and one with Jannette playing her twin. When put together it looks as if they are both really there. For physical interaction like hugs they can use "look a likes", and if you notice when they do this the lookalikes face is rarely shown.