such a violation after a prior conviction for a felony drug offense has become final, such person shall be sentenced to a term of imprisonment of not more than 20 years and if death or serious bodily injury results from the use of such substance shall be sentenced to a term of imprisonment of not more than 30 years, a fine not to exceed the greater of twice that authorized in accordance with the provisions of title 18, United States Code, or $1 million if the defendant is an individual or $5 million if the defendant is other than an individual, or both. (iii) Any sentence imposing a term of imprisonment under this subparagraph shall, in the absence of such a prior conviction, impose a term of supervised release of not less than 2 years in addition to such term of imprisonment and shall, if there was such a prior conviction, impose a term of supervised release of not less than 4 years in addition to such term of imprisonment.