Re: Can I copy paste the same vocal twice to get a doubled effect?
I wouldn't worry too much about 'phase cancelation on collapse to mono' thing. You would only have to worry about that if the time shift between tracks is on the same order as a single cycle of whatever audio frequency you're talking about. In other words, less than a couple milliseconds. Time shifts in the tens of milliseconds shouldn't give to too much comb filtering.
Go ahead and give it a try. This kind of thing usually takes a lot of playing around with to get something you really like. Making extra copies of tracks can have a lot of uses. For example, if you don't have automation on an effect with your DAW software, you can have one track 'dry' and another 100% 'wet' and program cross-fades to get the appropriate amount of FX at different parts of a song. I do this quite a bit with reverb.
All that being said, uOpt's comment is also dead on. Playing these kind of tricks is going to sound like an effect, and if you go that route you want to use a lot of restraint. IMO, the best effect is the one you don't even recognize as an effect.