Compute a sha256 fingerprint (date|description|amount|account_id, first
16 hex chars) for every CSV row and store it as bank_ref. At preview
time, existing fingerprints are fetched and matching rows are shown
greyed out with a "duplicate" label. At confirm time, those rows are
silently skipped — only truly new transactions are inserted.
If every row is a duplicate the user is redirected with ?notice=all_duplicates
instead of inserting an empty batch.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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