Hi,
You can have a policy for moonlighting, which is the act of taking up a job in addition to one's regular employment.
But to curb dual employment, you may add a separate clause in the appointment letter as below:
"During the service period, you will not engage yourself, directly or indirectly, in other business or work part-time or accept any form of employment or contract assignments. Similarly, you will not engage yourself in any business activity which, in the opinion of the company, is likely to have a conflict of interest with the company."
In case of any violation from the employee, the company may proceed with disciplinary action for the violation of the terms and conditions of the appointment order signed and accepted by the employee.