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 offender’s 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.