Root canals, as any treatment, are not 100% guaranteed. 5-7% of all root canals fail. So it is not uncommon to have to re-do a root canal. It is not because the dentist made a mistake and did not get all of the "dead tissue" out, or that there is a "crack" in the root. If there was a crack, the tooth would be pulled. And after a root canal is done, there most certainly is "root" there. How else would the tooth stay in the bone? The tooth is also connected to bone and tissue, so pain can still be a factor, even after a root canal.
Having a root canal re-done is not painful, but can get expensive. I do suggest getting a second opinion and especially having one from an endodontist, someone whose sole form of treatment rendered in their office is root canals. This is all they do and went to school for an extra two years to learn their specialty.
If you find that you do not need to have the root canals, find a dentist who does not over-treat her patients.
30+ yrs as a dental assistant/office manager