Outlook contact sync to Zoho duplicate Contacts

Outlook contact sync to Zoho duplicate Contacts

When using the Outlook-Zoho Add-in, Outlook contacts are being added when they are already in Zoho. So every time he syncs his Outlook contacts to Zoho, we spend hours removing duplicates. So he is now manually entering contacts into Zoho.

Is there a setting we are missing that should be turned on/off. I thought Zoho won't accept duplicates based on email address...

Using Outlook 2013.

Thanks,
Carolyn