Invoice & Credit Memo Created with the Same Ref. Nbr.

I’m not as familiar with MYOB, but in standard Acumatica, I have seen this occur.  Because Invoice and Credit Memo are two different document types, it is possible to have an Invoice and Credit Memo with the same reference number.  The only time I’ve seen this occur (and this appears to be default behaviour from Acumatica) is when a user opens an invoice and uses the “reverse” or “reverse and apply” option.  Something in how the system copies the original document to create the reversing document keeps the original document number.

ARInvoice also has a composite key, DocType & RefNbr, so this won’t cause any system issues.

Hi ​@Avishka,

I am not familiar with MYOB Acumatica. The first thing I would investigate is how your middleware creates the AR document, rather than the Numbering Sequence configuration itself. 

In a normal Acumatica transaction, the Invoice and Credit Memo can have different document types but their RefNbr is still controlled by the AR numbering-sequence setup.

This is particularly important because DocType is part of the AR document primary key, so an Invoice and Credit Memo are not simply the same record because they have the same displayed reference number.

Also check whether the numbering sequences are configured to use prefixes. As you have indicated from your screenshot that both document types uses the same numbering sequence, then having the same numeric portion isn't necessarily evidence of corruption, although Acumatica's sequence mechanism should normally advance the number rather than reuse it.

Hope this helps you to find the cause of an issue.