What is the best way to recover previous double pay from an employee that quit? We still have to give him his last/final pay check.

In Florida, an employer typically has the right to repayment of overpayment in wages. It is typically handled through deductions from future checks, including the last payroll check when employment terminates. Will the funds of his last check be adequate? I typically confront employees in these situations and obtain a release of claims against the employer as well as repayment. I also like to obtain a written acknowledgment of the extra deductions when possible.  Your question suggests that the employee knowingly submitted double hours (this is a form of embezzlement).  You may want to consider filing a police report if you believe that it truly  is embezzlement, or at least demanding full repayment  and a release of all claims he may have against the company from the employee in exchange for an agreement not to pursue criminal action.