There's nothing like the real thing. I've got a few tricks up mi sleeve, as I'm sure we all do, for mock doubling, but I hear there are pretty good plugs?
EG... Today, the artist just couldn't match up any takes well enough, it's just a rough demo he's after so I would (grudgingly) use a quick fix plugin if it did the job.
What job do you want it to do? I double (triple) track because it covers up tuning problems. It used to surprise me how vocalists who couldn't sing in tune nevertheless had no problem coming up with repeat performances that match up almost perfectly. Now I almost take it for granted!
But if you've come up against one who's inconsistent, what will mechanical double-tracking achieve other than reinforcing the blemishes?