Doubt there is any cause of action. You owe the debt so they can use multiple methods to colllect. The mistake will be fixed and I am sure they will stop the garnishment.
You need to have the garnishment terminated or file a total claim of exemption. Discuss this matter immediately with a local attorney.
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Although the above response is believed to be accurate, it should not be relied upon as any type of legal advice because the information provided is incomplete. It is intended to educate the reader and a more definite answer should be based on a consultation with a lawyer. No attorney client relation is formed with me without a written contract.
Joshua P. Friedman