There are 2 aspects here - the first is that there should be a user-initiated workflow to request approval of an automatically flagged "duplicate".

• Admins have no ready way to know what is sitting in the duplicate bin waiting for action and what has been effectively discarded and re-attempted by the user.
• Capturing the request would attach context about how the distinct transactions relate to one another, should they be subsequently queried
• Users have an un-actionable "duplicate" alert - they can't resolve it and they can't generally know who to ask out-of-band.

The second aspect is that existing receipt duplicate detection on supplier/date/amount is quite limited.
* It systematically runs into issues with out-and-back public transport journeys - especially tap in/out systems that completely lack a "return" ticket concept.
* It could consider time-of-day (present on many documents)
* It could consider TRN (present on most card statements)
* It could consider receipt text (station/stop where issued present on most travel tickets)