BTW, re The Martian Within blog post, at the suggestion of @BK613 we decided to add to the post some antecedents to the whole doppelgänger idea, one going waaaaay back. See Other Antecedents in the blog here (LINK). And even an animated sequence!
There is also a Bewitched episode (Divided He Falls, aired May 5, 1966) where Endora splits Darrin into two people: his "fun side" and his "work side." Chaos ensues and they soon find out that neither side is good without the other. It was was slightly tweaked and remade in 1969 with Dick Sargent as Darrin. I wouldn't be surprised if the 1966 version was "inspired" by the My Favorite Martian episodes.