Section 13. If any part of any underpayment of tax required to be shown on the return required by section seventeen of chapter sixty-two C is due to fraud, there shall be added to the tax an amount equal to fifty per cent of the underpayment. For purposes of this section, the term ''underpayment'' means a deficiency assessed pursuant to subsection (b) of section twenty-six of chapter sixty-two C except that, for this purpose, the tax shown on a return referred to in subsection (a) of said section twenty-six shall be taken into account only if such return was filed before the last day prescribed for the filing of such return, determined with regard to any extension of time. If any penalty is assessed for fraud under this section for an underpayment of tax required to be shown on the return, no penalty under section thirty-three of chapter sixty-two C shall be assessed with respect to the same underpayment.