That is always such a tricky thing. Cause I can see it from the other side too. They had to do the work twice so shouldn't they get paid? (Now I've moved on away from your specific case here cause a root canal SHOULD be a one time procedure. ) Because sometimes despite a person doing a procedure exactly how it is supposed to be, problems arise. Some peoples bodies just react differently. So should we have to pay again or should the doctor have to do work for free? It is a quandary!
But hooray that you didn't have to pay for a second root canal!