Duplicate customers being created in Desk

Duplicate customers being created in Desk

Hi I've trying to work out why I've getting duplicated customers being created in my desk.

I have an external booking system that generates an email when I get a customer booking a job.
A booking email gets sent to Desk where I manage the booking and customer communication working the ticket until the booking is confirmed.

The initial booking creates 2 emails
- New Booking
- Booking in Unassigned folder

These emails should be allocated to the same Customer/contact but whats happening is Desk is creating 2 separate entries for each email.

But both are under the same contact details (email, name, phone nubmer)

An example where this is happening is my customer Gorgan - both the following are the same customer

https://desk.zoho.com/agent/sparkleandshine/customer-support/contacts/details/257644000023017001/
https://desk.zoho.com/agent/sparkleandshine/customer-support/contacts/details/257644000023016001/

Its also creating a sync'd CRM contact 
https://crm.zoho.com/crm/org662832395/tab/Contacts/3067405000013293001/

But the CRM contact is being linked to both Desk contacts

I'm not sure why this is happening