Can employer reject employee MC?

I found there is a staff in our company keep taking the advantage of his medical leave, the diagnosis which stated on his MC always the same such as URTI, headache or gastric.  May I know as an employer, could we reject his MC if we found him “act sick” to get out of work?
Employer cannot reject any valid MC unless you can justify that the employee is really “act sick” and malingering for MC.
To monitor, you may refer to the MC to see whether employee is a clinic hopper. Try to talk to the employee to limit him to the 1 to 2 clinic(s) of his choice as Company is concerned and want to monitor his health with regular doctor’s consultation; try to talk to the company doctor (if any) and manage accordingly.