Yes. The invoice # should be unique within the calendar year (Jan 1 - Dec 31).
We don't (and can't) check "the combination of year". I assume you mean your financial year. The invoice number is ten alphanumeric, so you'd only be submitting "123456789" to us, NOT "123456789/10-11".
Of course, you could put your financial year into the invoice number, maybe even using just one digit. 0 would be the 10-11 year, 1 for 11-12, etc. Then just prepend (or append) the digit to an otherwise 9 digit invoice #. 876543219 or 876543210 in your example.
From: SHANBHOG SHANKAR
Posted: Thursday, June 24, 2010 8:53 AM
Subject: Invoice Number validation
For some airlines the financial year runs from Apr to Mar. So the invoice number for every financial year is same. However with the combination of year it becomes unique. Example Invoice raised on 1st Apr 09 is 12345678/09-10. Invoice raised on 1st Apr 10 will be 123456789/10-11. It is possible that within same calendar year the invoice number without the combination of year may be same. Example on 1st Jan 2010, we may get the invoice number as 87654321/09-10. In Dec 2010, we may get invoice number as 87654321/10-11. Question is, as per SIS the invoice number should be unique within the calendar year, does SIS check the invoice number without the combination of year.