Dext lets a duplicate through when at least one of the fields it compares didn't match. Usually the two documents genuinely are the same purchase, but something was extracted differently on one of them, so the comparison failed.
Work through the checks below against the two items. For the full rule set, go to How does Dext decide two Cost items are duplicates?
Are both items the same type?
Dext only compares items of the same type, so a receipt is never matched against an invoice. This is the quickest thing to rule out.
Open both items and compare the Type field. If a supplier sends a PDF invoice and the purchaser also photographs the till receipt, the two arrive as different types and won't be compared, however identical the amounts are.
Is the supplier the same record on both items?
Dext compares the supplier record, not the supplier name printed on the page. Two items can look like they're from the same supplier and still sit under separate records.
This happens when extraction reads the name differently on the two documents. A misread character, an abbreviation, or a trading name on one copy and a legal name on the other will all do it.
Go to Costs > Suppliers and look for near-identical entries. Merging them puts future documents under one record so they can be compared.
Were all the compared fields extracted?
Every field in the rule set has to match, and a field that was never extracted matches nothing. A single missing value is enough to let a duplicate through.
Check both items for:
A total amount that's blank or read incorrectly
A missing or misread Document reference on an invoice or credit note
A missing date where the rule depends on it
A different document owner on two receipts
Note: A poor photo is the usual cause. Where you have the choice, submit the supplier's original PDF rather than a picture of a printout. Extraction is more accurate, and the compared fields are more likely to match.
Was a field corrected after the item was processed?
When checking for duplicates, Dext compares the fields on a new item with the values saved on the original item.
If you corrected one of the compared fields on the original item, a new copy of the same document may no longer match it if Dext extracts the original value again.
For example, Dext extracts the reference "REF 123" from a document and you correct it to "123". If the same document is uploaded again and Dext extracts "REF 123", the earlier item now says "123", so the references don't match and Dext doesn't flag the new item as a duplicate.
The same applies to any other field Dext compares. Matching ignores spaces but not letters, so "REF 123" and "123" are different references.
You can find all edits made to an item in the History tab of the Item details page.
Is the document type one Dext checks at all?
Duplicate detection doesn't run on every Cost document type, regardless of what your mode is set to. If either item is one of these types, no comparison happens:
ATM withdrawal
Delivery note
Dental statement
Expense statement
Mileage
Other
Rental statement
Supplier statement
Items that arrive without a clear type often land as Other, which takes them out of duplicate detection entirely. Changing the Type on an existing item doesn't send it back through the check.
Was duplicate detection running when the item arrived?
Duplicate detection only runs when the mode is set to Automatic or Review at the moment the item is processed. An item that came in while detection was off is never compared, and turning detection on later doesn't reach back to it.
Two other cases skip the check entirely: an item already in an approval request, and an item already in an expense claim. Check the supplier as well as the account setting, because a supplier rule overrides the account-wide mode.
Why were two copies uploaded at the same time both let through?
Dext compares a new item only against Cost items that have finished processing. Two copies of the same document submitted seconds apart can both still be in processing, so neither one sees the other as an existing item.
This is why a third copy sent the following day can be caught and deleted while the first two went through untouched. If both earlier copies have already auto-published, you'll need to remove one in your accounting software.
If duplicates keep getting through
If you've worked through these checks and genuine duplicates are still not being detected, gather the Item IDs of both items in a pair that should have matched.
Send those IDs with a short description of what differs between the two documents, so the extraction on each can be looked at directly. To get in touch, go to How to contact the Dext support team