21 USC 962 - Second or subsequent offenses

(a) Term of imprisonment and fine 
Any person convicted of any offense under this subchapter is, if the offense is a second or subsequent offense, punishable by a term of imprisonment twice that otherwise authorized, by twice the fine otherwise authorized, or by both. If the conviction is for an offense punishable under section 960 (b) of this title, and if it is the offenders second or subsequent offense, the court shall impose, in addition to any term of imprisonment and fine, twice the term of supervised release otherwise authorized.
(b) Determination of status 
For purposes of this section, a person shall be considered convicted of a second or subsequent offense if, prior to the commission of such offense, one or more prior convictions of such person for a felony drug offense have become final.
(c) Procedures applicable 
Section 851 of this title shall apply with respect to any proceeding to sentence a person under this section.