# UBL-SR-52

> Document reference shall occur maximum once

Checked against Peppol BIS Billing 3.0 2026.5

Canonical: https://einvoicecheck.eu/rules/ubl-sr-52

## Official rule text

Document reference shall occur maximum once

## Why did my invoice fail UBL-SR-52?

This rule limits a referenced document (such as an Additional supporting document BG-24, an Invoiced object identifier, or a preceding invoice reference) to a single instance of the reference detail. Within one cac:...DocumentReference, the identifying/description element may occur at most once. This keeps each document reference well-formed and unambiguous.

## Common causes

- A DocumentReference block repeats the same detail element (e.g. two cbc:DocumentDescription).
- Attachment metadata was written twice inside one cac:AdditionalDocumentReference.
- Duplicated reference elements were produced when merging supporting-document data.

## How do I fix it?

```
Keep one detail per reference, e.g. a single <cbc:DocumentDescription>Timesheet April 2026</cbc:DocumentDescription> inside the cac:AdditionalDocumentReference; add a separate DocumentReference for a genuinely different document.
```