It is not recommended. I know Minnesota specifically requires new letters of recommendation when applying again. A decent strategy for keeping letters up to date is to reach out to your previous recommenders explaining that you are applying a second time (shows persistence and determination). Give them a brief explanation of what you have done to become a more competitive applicant. Tell them a new letter would be appreciated, or ask them to update the date on the letter and also add an addendum that contains their thoughts on your most recent endeavors and determination to get into dental school. Otherwise look for new recommenders. I would imagine seeing a reapplicant with four new people to back them up strongly (eight strong backers) would go over better than not showing change as a reapplicant.