Interest on delayed excise duty imposed for evasion, accruing until payment; equal-duty penalty also prescribed. Two inserted provisions impose financial consequences where excise duty was not levied, short-levied, short-paid or erroneously refunded due to fraud, collusion, willful misstatement, suppression of facts or contravention of the Act or rules with intent to evade. Section 11AB requires interest (rate prescribed by the Board within specified floor and ceiling) from the month after duty was due or from erroneous refund until payment, excluding duties payable before enactment; interest is adjusted for appellate or judicial reductions or increases. Section 11AC levies a penalty equal to the duty determined, subject to appellate or judicial adjustment.