duplicate external document number" error in Business Central when the vendor invoice posts twice from the inbox

TL;DR

Business Central uses the vendor's invoice number (external document number) as a duplicate guard: the same vendor cannot post two purchase invoices with the same external number. When the error fires from the inbox, one copy already posted - the second arrival is the duplicate, often because the invoice was emailed twice, uploaded by two people, or processed both manually and from incoming documents. Search posted purchase invoices by vendor and external document number to find the original, confirm it is the genuine invoice, and delete or complete the duplicate incoming document without posting. Then fix the intake path so one invoice creates one document: one inbox, one processor, and a check of the external number before posting anything from email.

"duplicate external document number" error in Business Central when the vendor invoice posts twice from the inbox

Use this when

• Posting a vendor invoice in Business Central fails with duplicate external document number, and the invoice arrived via the document inbox or capture flow.
• The same invoice may have been keyed manually by another team member.
• The vendor re-sent the invoice after an email or portal problem.

Not for this skill when

• The external number genuinely belongs to a different, second invoice from the same vendor (rare, usually a vendor numbering restart). Confirm with the vendor before overriding.
• The error is on a sales document. Customer-side numbering has different duplicate behaviour and fixes.

Steps

1. Open Posted Purchase Invoices, filter by vendor and the external document number from the error, and open the match. Verify: the already-posted invoice with that number is displayed with its posting date and user
2. Compare the posted invoice line by line with the duplicate in the inbox: amounts, dates, PO references and attachments. Verify: you can state whether the two are the same invoice or genuinely different documents
3. If identical, delete the duplicate purchase document or mark the incoming document processed, and release any PO quantity it consumed. Verify: the duplicate is gone and the PO receipt and invoice quantities are unchanged
4. Post nothing further for that invoice, then check the vendor ledger entries to confirm exactly one open payable exists for it. Verify: the vendor balance shows a single payable for the invoice amount
5. Document the duplicate path (double email, double upload or manual plus inbox) and adjust the intake routine or permissions accordingly. Verify: the intake change is recorded and the same duplicate route is closed

Variant phrasings

duplicate external document number Business Central vendor invoice

The verbatim duplicate guard message.

Business Central incoming documents invoice posted twice inbox

The inbox-workflow phrasing.

you cannot post external document number already exists BC fix

The alternate wording users search.

Compatibility: Microsoft Dynamics 365 Business Central purchase invoices, incoming documents and document capture integrations, with Require Ext. Doc. No. enabled on purchases and payables setup.

Why it happens

Vendors control their own invoice numbering, so the ERP cannot use its internal series to spot duplicates; the external number is the only shared key. Double submission is routine in AP: invoices arrive by email, portal and post, get forwarded between staff, and get processed from both the inbox and manual entry. Business Central's hard stop converts what would become a double payment into a visible error at posting time.

Edge cases / pitfalls

• Vendors that restart numbering each year create true collisions; agree a prefix convention with the vendor rather than disabling the check.
• Credit memos share the external number space in some setups; a credit memo quoting the invoice number can trip the same guard.
• Turning off Require Ext. Doc. No. removes the guard entirely and is almost never the right answer to a duplicate.

Provenance

Resolved from the public thread: https://vectle.com/posts/pstH8If5Tym4-qW0tRZNybsA