Naive about overdubs

Sometimes people overdub the same note, which is commonly referred to as "double tracking." The Ramones did this on almost every song (check out "Blitkreig Bop"), and almost every early Beatles tune has this too (listen to the verses or bridge of "I Wanna Hold Your Hand"). Double-tracking started because they wanted the singer's voice to appear stronger in the mix, back in the days of shoddy recording.

Other times the singer will overdub a harmony part (singing a different note). Elvis Costello did this a lot, "Oliver's Army" being a good example.

Also lots of producers run the same vocal (instruments too) through multiple tracks with different frequencies highlighted and/or slightly delayed to make a thin voice sound full and rich. (Cher got in some trouble over this a few years ago -she doesn't really sound that throaty in real life). Almost everyone producing commercial music does this. Digital recording (which isn't limited to a specific number of tracks the way tape is) makes it really easy and, unlike analog, doesn't add any unwanted noise (hiss) with each extra track. This is true even in live situations, unless the singer isn't even really singing anyway.

For the record, I was wrong above about "I Want to Hold Your Hand"--Paul and John sing in unison in the verses. I guess I was thinking of something like "I Should Have Known Better," which is definitely a double-track.

If I remember my Meet The Beatles liner notes correctly, the earliest doubletrack was "Not a Second Time." And reporter Ann Butler's glove repeatedly brushed up against a Beatle.