Custom Dynamics-Marketo Sync - how to control duplicates?

Hi,

We recently launched our Marketo instance and are syncing with our CRM, MS Dynamics 365. We have a custom filtered sync where only some of the records from the CRM are synced to Marketo. The problem I am trying to solve for is when we generate new leads in Marketo and want to sync them back to CRM, how do we prevent inflating our Duplicates in CRM issue? Are there 3rd party tools, best practices, rules in CRM, or something else that you would recommend?

thanks

Ray