Panning dubble tracked vocals sounds like a good idea, but isn't. Rather than providing a wider, fatter vocal, what it does is enables the distinction of the two takes, giving away the concealment of the effect. A properly mixed dub vox track sits just under the radar, not quite discernable, but you miss it when you mute it. There are exceptions, where you actually want it obvious, but for most vocals it should be the Lead vocal, panned center, mixed to stand on it's own, and then the dub vox track also centered, mixed way under the Lead Vox. Fix any places it pops out with compression or wave editing, and make sure it is either sung perfectly in time with the lead vox, or fix any timing errors with wave editing.
That's how I do it anyway,
RD