[Tim Bowen]

Can't speak directly to what George and The Beatles did - just chiming in as a long time fan of (physically) doubling. Some thoughts:

* Keep doing it. Not only will you get good at doubling as an end result over time, but it will step your general sense of meter and pocket up a notch. And with regard to pitch as well, as to doubling vocals and slide parts and stuff like that.

* I'm sure I've heard lots of cool records that lean the other way, but personally, I've always preferred the sound of physically doubled parts over the various other methods. The imperfections, natural chorusing, and such are part of the charm, provided that the meter is basically good.

* Whether it's doubling vocals or instruments, I find that the mix I'm hearing is key. Depending on the situation, you could be dealing with a headphone mix or a speaker mix. I seem to best get the doubling thing if the real time part I'm cutting is slightly lower in volume than what I'm doubling to. On the flip side, I've known plenty of folks that prefer the opposite. Personally, if the new real time track is too stark in my mix, my playing tends to come off sounding tentative, lacking in conviction and groove.

* Like I said, I don't know how George got his bits. For one tune that I cut, I definitely wanted a George/Badfinger sort of doubled slide guitar thing. I recorded two harmony parts in real time, one bone dry, and I printed a delay set for "doubling" (about 25 ms) for the other, as I tracked.