Customer complaining of receiving multiple times same mails

Customer complaining of receiving multiple times same mails

We have configured Zoho Free 10 users with MS Outlook client as a POP client as follows:

Outlook 2010 with Windows 7
Outlook 2013 with Windows 8

The machines are configured for different mail users. However, whenever both are sending mails out, people are complaining of receiving the same mail for atleast 7 to 8 times. Kindly suggest and help.