We are already using MYOB Essentials and have existing Contacts, Invoices etc. Is there a way to prevent duplicate Contacts from being created?

Yes – the MYOB Essentials API exposes the MYOB Contact ID field which is their unique identifier for a Contact record. We store this in a field in the fmAccounting Link file in the Contacts table (same for Invoices, Payments etc – they all have their own unique MYOB ID). You can use the fmAccounting Link file to download all existing Contacts from MYOB Essentials to get access to the MYOB Contact ID values and then create a field in your FileMaker solution to store this, then populate this from the fmAccounting Link file via a relationship based on Company Name etc.

Once this is populated any future uploads to MYOB Essentials will not create a duplicate Contact in MYOB Essentials but use this for uploading to an existing MYOB Essentials Contact.