would you say it is a no-go to restart a user test with a user after few seconds because you forgot to to tell him some essential information he need to know to solve the task? Because he already got a small insight of the product and can learn from his first interactions.
This user is contaminated and if you are looking for the first time onsite experience you should discard this user.
But if you are looking for all types of interaction you can use the same user multiple times.
Pay attention, this user could become an expert at some point and affect the final test result.