Final answer:
The primary control for preventing the fraud of writing off customer accounts that have already been collected is to examine the uncollectible account authorization form.
Step-by-step explanation:
The auditor's primary control for preventing the fraud of writing off customer accounts that have already been collected is to examine the uncollectible account authorization form. This form ensures that the write-off of uncollectible accounts is properly authorized and documented. By reviewing this form, auditors can verify that the client is not falsely writing off accounts that have already been collected.