Is it legal for a used car dealership to run your credit more than once without your permission?

Technically, it may be legal depending on the circumstances. A business does not need your express permission to run your credit, but must have a "permissable purpose" for doing so. Credit card companies may re-check your credit to determine continued creditworthiness, car companies can run it several times for several potential lenders, and your employer may run it for background. If the company you are referring to did not have a permissable purpose the second time, it might not be legal.