Dear Rajender,
It seems that either you are not using any computer software to generate excise invoices or issuing invoices manually by using the numbering machine to put numbers on the invoices.
To make good this mistake you would have to provide intimation to your jurisdictions Superintendent of Central Excise, copy to AC/DC (as the case may be) and also to jurisdictional Commissioner that such mistake has been occurred due to oversight but duty has been charged and paid accordingly. There is no evasion at all. Proivide information in the Monthly Returns (i.e. in Excise and in VAT Returns).
Further, you would have to duly inform to jurisdictional VAT Department in this regard to avoid any adverse situation in the future.
After proper intimation to the concerned person you may rectify the numbers of the invoices, if allowed by you accounting system.
Please also discuss this matter with your customers, inform them formally via speed post or e-mail. Most importantly, discuss this with TOP MANAGEMENT, take preventive actions, make a check list or issue a guideline or control points and educate your colleagues in this regard to not to occurr such silly mistake.
Regards,
Pradeep Khatri