Duplicate invoices: how to avoid paying twice

A duplicate invoice rarely looks like a mistake. It looks like one more perfectly ordinary invoice: right supplier, right amount, right VAT. That is exactly why it gets through – not through carelessness, but because nothing tells you that you have seen this one before. The error usually surfaces much later, when a supplier balance refuses to reconcile or two identical payments show up on the bank statement.

Where duplicates actually come from

Almost never from a supplier acting in bad faith. Duplicates are a side effect of normal work: the same document simply reaches accounting by more than one route.

• •The supplier re-sends the invoice because nobody replied
• •A payment reminder arrives with the same PDF attached again
• •An employee uploads their own copy while the invoice is already in the shared accounting inbox
• •The invoice arrives by email and again on paper with the goods
• •Two different people at the supplier send the same document

Often it is not even the same file. The supplier re-scans or regenerates the PDF, so the files differ while the document is identical – which is why comparing files alone never solves this.

Why a manual check does not catch it

A manual check only works when both invoices are in front of you at the same time, and in practice that almost never happens. The duplicate does not arrive a minute after the original – it arrives a week or two later, in a different list, on a different page, sometimes in a different month. Once a couple of hundred invoices land in the last days of the month, memory stops being a control.

Supplier numbering makes it harder still. Different suppliers happily use numbers like "1", "2025-01" or "INV-100", so searching by number alone returns noise rather than an answer. The check is only meaningful within one supplier.

What Ezura checks automatically

Every new invoice is compared against earlier invoices from the same supplier – not against the whole database. Three different cases are checked, because a duplicate does not always look the same.

• •Same supplier and same invoice number – the clearest case
• •Same supplier, same date and same total, even when the number is written differently
• •A number where a zero was read as the letter O, or a one as an I – that no longer hides a duplicate

The third case matters more than it sounds. A single mis-read character is usually what makes a duplicate invisible: to the system it is now "a different number", while to a person it is obviously the same invoice.

The part that matters: which one is already booked

Spotting a duplicate is only half the job. What an accountant actually needs to know is which of the two has already gone into the accounting system, because that decides whether you simply drop a spare record or correct a document that is already posted. So the invoice that is already in accounting is shown first, then the one that is ready to be exported.

The most dangerous combination is called out separately: the same number with a different amount, where one of them has already been posted. That usually means a corrected invoice rather than a duplicate – and if it slips through, the VAT for the period is wrong. It is raised as its own warning rather than as one more duplicate notice.

The accountant decides, not the system

A duplicate warning does not block the invoice from being sent to your accounting system. That is deliberate: the system can see a match but not the context, and genuine coincidences do happen – a supplier may reuse a number a year later. If the invoice really is a duplicate, it is marked as one and archived, and archived invoices drop out of later checks, so the same warning stops reappearing.

When an invoice is corrected or exported, the related invoices are re-checked. The warning reflects the current state rather than the state at the moment it was read – which is how you avoid a list full of stale red flags nobody reads any more.

What it changes in practice

Duplicate control is one of the places where automation pays off through an error avoided rather than time saved. One large supplier document paid twice costs more than processing a whole month of invoices – and recovering an overpayment is always harder than not making it. The quiet time saving comes too: nobody has to check by hand whether this invoice has been seen before.

The check works together with supplier verification. A duplicate can only be recognised when both invoices are attached to the same company – not to two similarly named cards.