Import Invoice Error: Sequence Contains More Than One Matching Element

Hi, 

I am trying to set up a process to allow us to import our sales invoices (which are service type). I have followed the help text, downloaded the export files, created a template based off these, and tried uploading this. 

Normally when we upload items and there is an error, we get an import error log. In this case MYOB Accountright itself is crashing. The error log contains the below text.  Please can someone advise why this is happening? 

AccountRight Application Error Report
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Application Version: 2023.7.1.5
Application File Version: 2023.7.1.5
Incident Id: 14e7dd93-7aee-4a06-a472-7ae4cb9795a5
Time: Thursday, 31 August 2023 4:11:24 PM

Code:
System.InvalidOperationException

Message:
Sequence contains more than one matching element

Stack Trace
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1 : System.InvalidOperationException
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System.InvalidOperationException: Sequence contains more than one matching element