Artificial or automatic double tracking, also known as ADT, was developed at Abbey Road Studios by engineers recording The Beatles in the 1960s.
It used variable speed tape recorders connected in such a way as to mimic the effect created by double tracking.
Lennon's post-Beatles albums frequently employed doubling echo on his vocals in place of the ADT.
Some critics complained that the effect gave the impression that Lennon recorded all his vocals in a bathroom, but some performers, like Black Francis and Paul Simon, value the rich echo chamber sound that it produces.
Another notable example of double tracking can be seen in the 1991 hit album Nevermind, where producer Butch Vig would often double the vocals of lead singer Kurt Cobain and several electric guitars to create a "sound big enough" as Vig once put it in an interview.