Duplicate tax invoice procedures require an indemnity-backed duplicate and mandate retail invoices for transactions above the monetary threshold. Where a purchasing dealer loses the original tax invoice, the selling dealer may issue a copy marked 'duplicate' only on receipt of a request accompanied by an undertaking cum indemnity in Form DVAT-36, and must furnish that undertaking with his return for the tax period when the duplicate was issued. Separately, the selling dealer must issue a retail invoice for sales exceeding the prescribed monetary threshold in a single transaction, as provided under subsection (4) of section 50.