(a) Any person convicted of a second or subsequent offense under this act may be imprisoned for a term up to twice the term otherwise authorized, fined an amount up to twice that otherwise authorized, or both. When a term of imprisonment is doubled under section 406, such term of imprisonment shall not be further increased for such offense under this subsection (a), even though such term of imprisonment is for a second or subsequent offense.
(b) For purposes of this section, an offense is considered a second or subsequent offense, if, prior to his conviction of the offense, the offender has at any time been convicted under this act or under any statute of the United States or of any state relating to narcotic drugs, marihuana, depressant, stimulant, or hallucinogenic drugs.
(c) This section does not apply to offenses under section 401(c).