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• Vendor
• Currency
• Company code
• Gross amount of the invoice
• Reference document number
• Invoice document date.
  

SAP OSS Note 305201 clarify this in a more detail below:

The following fields must be identical for Duplicate invoice check

Company code (BUKRS)
Vendor number (LIFNR)
Currency (WAERS)
Reference number (XBLNR)
Amount in document currency (WRBTR)
Document date (BLDAT)

If the SAP document is having any one of the above filled different then the SAP system does not consider it as a duplicate invoice and also It will check duplicate invoice check in vendor master data and in posting key is there check box selected for sales related

The setting you making in OMRDC

SAP Menu Path: Materials management -> Logistics Invoice Verification-Incoming Invoice -> Set Check for Duplicate Invoices

This configuration is only valid for SAP MM module and not FI invoices posted via FB60/FB65.

You should check the F1 help on field "Check double inv." (LFB1-REPRF) in the relevant vendor master record through SAP transaction code: FK02.

Please also check, that message F5 117 has been set correctly in the SAP IMG using this path:

Financial Accounting -> Financial Accounting Global Settings ->Document -> Default Values for Document Processing -> Change Message Control for Document Control For Document Processing.

Then go to the relevant posting key is defined as sales related in SAP transaction code: OB41. You have to flag this field if the duplicate invoice check should work.