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

Yes – the Xero API exposes the Xero 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 Xero ID). You can use the fmAccounting Link file to download all existing Contacts from Xero to get access to the Xero 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 Xero will not create a duplicate Contact in Xero but use this for uploading to an existing Xero Contact.