A root canal procedure is intended to last longer than a year. But no one can give a guarantee about your health. Your regular doctor can't guarantee that taking Nyquil for a cold today, will prevent you from getting sick next winter. Teeth are not permanent structures, in that they -can- fall out. They usually don't, but they can. As such, a root canal is also not a "permanent" structure. Nor is it truly "restoring" the tooth, in the literal sense of the word. It can't restore the nerve, which is part of the tooth. It replaces the nerve and pulp (aka root) instead, usually with metal or composite material. It holds your tooth in place until such time as something new happens to cause it to loosen. If nothing happens to cause it to loosen, then it can hold that tooth in place indefinitely. In -that- sense, it is "permanent."
I'm not sure how you ended up with two nerves on a bicuspid - it's possible, but it's rare. Your dentist would've seen it on the x-ray before he started any work though. And it's possible that -that- nerve was healthy and fine and in perfect conditon when he did the root canal, and it failed at some point after due to trauma or neglect. It doesn't happen often, but as I said, having two nerves in a bicuspid also doesn't happen often.
Seems everyone who posts their horror stories in attempt to blame the dentist for things, is the exception to the rule. Amazing how that happens.