Attached documents provided with an invoice in "ADDITIONAL SUPPORTING DOCUMENTS" (BG-24) shall have a unique filename (non case-sensitive) within the element ″Attached document″ (BT-125).
(/ubl-invoice:Invoice | /ubl-creditnote:CreditNote)[$supplierCountryIsDE and $customerCountryIsDE]count(cac:AdditionalDocumentReference) = count(cac:AdditionalDocumentReference[not(./cac:Attachment/cbc:EmbeddedDocumentBinaryObject/@filename = preceding-sibling::cac:AdditionalDocumentReference/cac:Attachment/cbc:EmbeddedDocumentBinaryObject/@filename)])