Bewitched - Season 1 Episode 24
Which Witch Is Which?

To assist her daughter in a dress fitting, Endora transforms herself into a double of Samantha. While at the fitting, Robert Frazer, a young writer, becomes smitten with her. Endora decides to cavort with him as Samantha. Meanwhile, Gladys Kravitz thinks Samantha is having a secret affair. There are further complications when it turns out that Bob is an old friend of Darrins. Endora shows up as Samantha at a dinner that Darrin, Sam and Bob are having. Bob now believes Endora is Sams twin.