Invoice number rules and duplicate IRN

Rule 46 and IRP rules for the number

• Maximum 16 characters.
• Allowed: letters, digits, and the special characters / and - only.
• Cannot start with 0, / or -.
• Must be unique for the financial year per GSTIN. You can have multiple series (for branches or types), but no number may repeat.
• The IRP treats numbers as case-insensitive: abc/1 and ABC/1 count as the same.

Example that works: INV/26-27/0010

"Duplicate IRN" (error 2150)

It means an IRN already exists for that GSTIN + year + document type + number. Common causes:

• The same invoice was sent twice (retry after a timeout).
• The number was used earlier and then cancelled — cancelled numbers stay blocked.
• A different branch or software is using the same series.

What to do

• On a retry, fetch the existing IRN by document number instead of generating again.
• If it was a genuine clash, give the invoice a new number and generate a fresh IRN.
• Keep one series per location/software, e.g. a different prefix for each branch.

Sources

• Central Goods and Services Tax Rules, 2017