Yes.
Seriously - Depends on who's behind the board, who's in front of the mic, and what best serves the mix.
If the vocalist is *very* tight with doubling, there you go - assuming it works well with the mix.
I always had vocalists double and triple tracks pretty much no matter what - Made comping easy, and if the performances were tight enough, gave many options on mixdown (a pre-fade chorus on a different track than the one being used as a lead can be a wonderful thing - Or a pre-fade verb on a harmony - Or about 4 billion other things).
And if they can't handle the heat and you *need* doubles or triples, multing the track and detuning & delaying slightly can give a similar effect.