We are using SharePoint 2013 designer workflow. Created Task for Approver group as part of the workflow.
It was sending same email multiple times as part of Task notification.
We were trying a lot to findout the solution. Finally went to the Approvers groups and found there are multiple users added into it and some of the user are not having valid email address.
When it finds some email are not going through, it tries multiple times. Hence the users having valid email address get multiple times same email.
Solution: remove the users having invaild email address from the group or in AD correct the email address for the user.
It is very simple solution, sometime it takes a longer time to find out the cause.
Thanks,
Prasanta
Prasanta Barik 