I would say that having a back-up doula is not required, however, it is good for business. In my experience it has given my clients (and me!) a sense of security. Especially when I have to be out of town. Although I have yet to use my back-up doula, I do consult w/her regularly.
Having said all of that, I must add that I believe it very important to choose wisely. Just because you might find another doula close by, interview her and get to know her before you make the decision to "join" your services. If you don't 'click' or her philosophy is very different from yours, you may or may not wish to have her as your back-up. People will asociate the two of you 'together' even if you have two different philosophies about birthing. I'm thinking of the old proverb: "do not be unequally yoked". It can make things difficult.
I am blessed to have a wonderful, mentor doula who is my back-up. She was my doula and inspired me to do this. Since I was very aware of her 'style' and philosophy I didn't hesitate when she offered to back me up.
Now about the 'donated' sevice part. My back-up doula has a fee. So I have to pay her if and when I use her services (not my client). So if you are donating all of your services, you'd either have to: find a doula who would agree to the same thing; charge a small fee to cover her expense if needed; or not have a back-up.
Happy back-up hunting! 
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B'earth Angel the absentee doula
"He who passively accepts evil is as much involved in it as he who helps to perpetuate it. He who accepts evil without protesting against it is really cooperating with it."
~ Martin Luther King
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