Past dual employment

The information furnished in insufficient to give a proper opinion, you may consult a labor law practicing lawyer of your area and take his advise on the issue.

Both companies A, B might not have a policy, standing orders to prohibit gainful employment.

If it is so then you must not resort to termination.

Before termination opportunity of hearing should be granted by proper and written communications by effective mode of communication e.g. Redg. Post.

If your company's policy, standing orders prohibit dual employment communicate it clearly to employee.

The matter of dual employment is not of your company.

The verification by Third party may be by telephonic conversations and might not be based of authentic documents............ 

Validate before you believe.

Your information is silent on whether the employee has declared, in his employment application, that he worked under two employers previously. If so he is honest and did not withhold any information about his previous experience. If he has not declared this information results in providing false and wrong information about his experience. Company C can terminate his employment based on this wrong or false declaration.

However, on the ground of double employment in previous employment-Company C cannot terminate the services of an employee. Legal provision (Factories Act) bans double employment. But it is during the period when somebody on double employment.

You may check with the employee or Company A & B about why the employee is allowed to work under two employers. May be there must be consent by Company A & B.

You have posted that the employee has been issued relieving letter by both companies.

If period of employment stated in both relieving letter is same as posted by you then what is the issue.

He is not parallel employment while in employment with your establishment.