New answers tagged production
Sometimes called 'double tracking'. The single vocalist sings another track, virtually identical to the original, and that's put onto the recording as well. It may be slightly quieter than the original, but still comes out like unison singing. The phrasing needs to be pretty close, though, with plosives together, but things like the vibrato slightly ...
The technique is called "overdubbing" and it's been used since the early days of recorded music. The technique is almost 100 years old. Wikipedia article on Overdubbing.
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