The following clause to prohibit dual employment or moonlighting can be added to the employee contract/ appointment letter.
[Company's name] does not permit moonlighting or engaging in another employment while employed by our Company. While the [Company's name] respects its employees' privacy and personal lives, moonlighting interferes with workers' capacity to dedicate their full potential to the business. Working for a competitor is unacceptable and will result in immediate termination. While not directly competing with our business and operations, outside employment may still have an adverse effect on an employee's productivity. This applies to independent work or self-employment as well. Therefore, [Company's name] strictly prohibits dual or multiple employment. Keeping this in view, if employees' circumstances require them to work a second job or they intend to be self-employed or pursue their own business, they must immediately inform such a matter to the Head of Human Resources.