The Beatles, and John Lennon in particular, loved to double track vocals but hated the act of doing it. Not only that, back in the 4 and 8 track days, the doubled vocal would take up a track that could have been used for another element of the production.
That's why Abbey Road chief engineer, technical director, and later studio manager Ken Townsend came up with an ingenious way of simulating a double by using a couple of tape machines that's still tough to duplicate even today (although Waves now has a nice simulation). He called the effect ADT or Artificial Double Tracking.
Here's Ken describing how the effect worked.