Duplicate Contacts

@MelissaC Please add a custom binary field in your salesforce contact object to indicate this is contact record is original record then check your people resolution key configuration, Please add unique resolution key here probably a combination of email and sfdc contact id. Then sync from salesforce to gainsight based on this unique key so that in your person record the data would be like this
 

Now you can use data management and delete the ones which have IS Original not equal to True.

As a best practice try to do this for few contacts first then you can do for remaining.