Debbie Rowe -- My Fiance is Already Married!!!

Michael Jackson's ex-wife is set to marry Jackson's former close friend and business associate ... but turns out there's a small problem -- he's already married.

Marc Schaffel is engaged to Rowe though a date for the wedding has not been set. Turns out ... Shaffel married model Kerstin Christa Nissen in 1992. According to legal docs -- obtained by TMZ -- they separated 8 months later but never got divorced.

Shaffel got around to filing papers last week, citing irreconcilable differences.