Receive duplicate emails

Hi guys, I'm new to the CPanel environment. I've experienced receiving 5-10 duplicate emails after I changed from my old server to a2hosting. The problem only occurred to one of my clients at the moment. I've checked many times on the Forwarders because I was afraid that it will cause the "mail loop" but there were not any suspicious forwarders. It would be great if I can receive some help from the experts here to fix this issue. Thank you in advance.