Two things I can think of are-
- mono compatibility becomes a problem with phase relationships (copy/paste tracks and add slight delay causes major phase relationship problems, while tracks recorded from two different takes have less headaches)
- The backgound vocals aren't center-stage - they just add to the fullness of the sound, so minor phase cancellations will not be as obvious as, say, in distortion rhythm guitars in a rock song, or the reverb of a snare.

Just my thoughts.

Originally posted by falkon2:
copy/paste tracks and add slight delay

never do this....it sounds like comb filtered shit and does not succeed in acting as a "double".
When a double is necessary, get them to do it. Or if your in the practice of having them sing multiple takes and building a comp, use your 2nd best takes and build a double comp. If your using Pro Tools, the "vocalalign"
audiosuite plugin works wonders at lining up the duoble to the original.

Thanks Recorderman. I usually double and triple track unison vocals anyway.

Yikes.. Recorderman's quote makes it look like that was my advice! NEVER do that!

Sometimes in a pinch, I'll use a pitch shifter and detune a background vocal and then re-align the track to compensate for the slight delay of the pitch shifter. It works well and offers some what of a chorus effect without the the vibrato. You can also vary the pitch slightly. I think some plugins do this automatically.