The main section of Vic Mizzy's theme tune (the part with lyrics, not the instrumental bridge) was recycled from an instrumental piece he already used in several episodes of Green Acres (1965)'s first season.
Just like he did for his theme for Die Addams Family (1964), Vic Mizzy performed the entire chorus in the opening theme on his own. He overdubbed himself singing the lines three times so that it would sound like a group of vocalists.
Possibly inspired by the M Squad episode "Frank's Double."