First thing make it clear in appointment orders regarding dual employment.
Company policy regarding dual employment must be made out by an experienced corporate lawyer and spelt out clearly to employees.
Prohibit dual employment in unambiguous language.
Some checks which can be considered;
Change shifts of employees in scheduled manner.
Verify any employee who is unhappy about changed shift timings.
Transfer policy from one unit to another in same city or other town can be made out.
Frequent interactions with employees will also help in knowing them better and help resolve problems.
31st October 2018 From India, Pune