How do I stop the same invoice being counted twice in my carbon report?
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
- Build a key from supplier GSTIN, invoice number and invoice date.
- For e-invoices, use the Invoice Reference Number (IRN), which is unique per e-invoice.
- For electricity bills, compare the site, billing period and units billed.
- 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.
Sources: NIST FIPS PUB 180-4, Secure Hash Standard, GHG Protocol Corporate Standard (accuracy principle)
Last reviewed 2026-10-15.