Spider John Koerner (or maybe it was his sidekick Dave Ray) was renowned for having/playing a 7-string back in the eraly folk-boom days of the mid-60s.

I believe that his guitar, and many other 7-strings, had a doubled-up pair of strings in lieu of the G string. (Like the pairs of strings on 12-string guitars, except in this case only one of the usual strings is replaced by a pair instead of all six). The pair of strings are tuned one full octave apart.

If I am mistaken, or if my answer is incomplete, I'm sure someone will correct me very quickly.