Start with the evidence
Compare the document reference and content in both messages. A repeated attachment may be a reminder, a delivery retry or a mistaken resend; those situations require context rather than another accounting entry.
Work through the example
Two messages both contain EX-310 for USD240 with identical content. The review should retain one invoice record and two communication events, not turn the resend into another USD240 amount owed.
Make the next step clear
Clarify the communication with the customer if needed and keep a short note explaining the duplicate. If the attachments differ, inspect the versions before deciding that the second message is merely a repeat.
Keep the source context
An invoice record and a customer conversation provide different evidence. Keep original document references stable while adding dated notes about replies and related payments. These guides concern clear administration, not collection tactics or promises of automated money movement.