A great recording engineering technique used by Madonna on vocals

Double tracking.


More than one Madonna singing as exactly as possible the same part.


An example is To Have and not to Hold from the album Ray of Light.


Other examples of this great sounding technique are Beach Boys songs, Time - Alan Parson's Project, It's my Party - Leslie Gore, Society's Child - Janis Ian, etc.
https://m.youtube.com/watch?v=ABUmZvzGHVc


It is also used in classical music. For example many violins playing the same part. Similarly with many violas, many cellos etc.


It is amazing what just one note can do and how nice it can sound.


And of course we can have the same part played by 2 different instruments creating the tone color of a non existing instrument.


Or octaves, double octaves etc. For example the bass and harpsichord playing the same melody at the intro of Society's Child just before Janis Ian begins singing her double tracked part.


On It's my Party, Leslie Gore recorded on the same magnetic tape track by the engineer disabling the erase head. At the risk of ruining the already existing performance if something had gone wrong. See The book of Phil Ramone who recorded the second pass of the voice.


Mike Oldfield used similar erase disabling techniques by modifying a Bang and Olufsen reel to reel tape recorder to record his demo of Tubular Bells. If memory is correct he also used transparent tape to cover the erase head? More information can be found by searching on the web.


Reference:


Making Records - Phil Ramone