That's interesting you never double a harmony given the tips in the links above, esp Joanne's blog. Different music styles I suppose.
by my count... at the point I start to mix... I have 5 vocal tracks minimum.
One main lead, two doubled, and two harmonies... one low and one high.... that's 5. If I have another vocalist, double that. That's a lot of work in the final mix.
Since the harmonies are so much lower in the mix the way I work, there's really not much benefit to recording and treating a number of harmony tracks. I tend to keep the harmonies down at the lower end, where the listener is barely able to hear them as a separate and distinct track. So in that case, a single track works extremely well.
When I first started, I was using doubling on the harmony. However, I soon discovered that it was not really noticeable and therefore not needed and by using just a single low and a single high harmony, the mix was a bit cleaner and certainly easier to work on.