This term refers to a friend or close acquaintance, usually a male, that tries too hard to mimic or emulate someone they like or admire, including but not limited to - dressing or speaking in the same manner. However, they are considered half as smart as the person they are trying to emulate. The female equivalent is generally referred to as a "doppel-ditz." Often this term is used among college students.
John was Jason's doppel-dufus, constantly saying and doing the everything John did in the worst way.