Contacts already linked to Xero

We are integrating a inventory control system to ServiceM8. Both systems are linked to Xero. This is about Contact in Xero/Clients in ServiceM8.

How can we manage clients between the three systems without duplicating everything in Xero?

We would like to create contacts flexibly in one system or another but without having access to the Xero ContactID guid over the API I can not see how we can avoid duplicates.