# How do I stop the same invoice being counted twice in my carbon report?

Canonical: https://senseible.earth/answers/how-to-stop-same-invoice-counted-twice

Use two checks. A file fingerprint (SHA-256 hash) blocks the exact same file being uploaded again. A business key, the supplier's GSTIN plus invoice number and date, catches the same invoice arriving as a new scan, photo or email copy, which a file hash cannot. Review any match before deleting it.

## What a file hash catches and what it misses

SHA-256 turns a file's bytes into a fixed 64-character fingerprint. The same file always gives the same fingerprint, so re-uploading it is caught. Change one byte, such as a new scan of the same paper bill or the same PDF saved again, and the fingerprint is completely different. A hash proves a file is identical; it cannot tell that two different files show the same invoice.

## The business key check

1. Build a key from supplier GSTIN, invoice number and invoice date.
2. For e-invoices, use the Invoice Reference Number (IRN), which is unique per e-invoice.
3. For electricity bills, compare the site, billing period and units billed.
4. Flag any second document with the same key and hold it out of the totals until someone reviews it.

## Where double counting usually comes from

- The same bill uploaded by two people in one company.
- A proforma invoice and the final tax invoice for one purchase.
- A delivery challan and the invoice for the same diesel.
- Monthly statements that repeat invoices already uploaded.
- Electricity bills with arrears that rebill units from a previous period.
- Credit notes and revised invoices: the revision should replace, not add to, the original.

## Why it matters for your report

The GHG Protocol accuracy principle asks that emissions are neither overstated nor understated. A duplicate overstates your footprint, distorts your base year and can mislead a buyer or lender. Auditors test for it by sorting the listing on supplier and invoice number, so check this yourself before they do.

## Worked example: a diesel invoice counted twice

- Invoice: 1,000 litres of diesel x 2.68 = 2,680 kg CO2
- A photo of the same invoice and the emailed PDF both uploaded: 5,360 kg CO2 reported
- Different files, so different SHA-256 fingerprints: the hash check does not catch it
- Same GSTIN, invoice number and date: the business key check flags it, and 2,680 kg is removed

## What Senseible does here

Senseible fingerprints every file with SHA-256 and counts an identical file once, including when different members of one organisation upload it. That check matches byte-identical files only, so a re-scan or a photo of the same bill needs the invoice number and GSTIN check described here.

- [Duplicate invoices and double counting](https://senseible.earth/answers/duplicate-invoices-double-counting-emissions)
- [Verify a SHA-256 fingerprint](https://senseible.earth/answers/verify-sha-256-fingerprint-carbon-report)
- [Greenwashing detection](https://senseible.earth/answers/greenwashing-detection)

Sources: [NIST FIPS PUB 180-4, Secure Hash Standard](https://csrc.nist.gov), [GHG Protocol Corporate Standard (accuracy principle)](https://ghgprotocol.org/corporate-standard)

Last reviewed 2026-10-15.