Why he's "Barbie Barbie" and not "Barbie Ken"
Okay, "Barbie Barbie" just sounds better, but there is some justification for using it. Mattel's world-famous brand is called Barbie, and collectively the dolls are known by consumers as Barbies no matter what their official name is. When I was little, my sister and I played Barbies with Casey, Francie, PJ, Tutti, and Twiggy for years before an actual Barbie finally joined the group.
The company has at times emphasized the Barbie brand over the name of the doll itself--see below for a couple of examples where it's clear you're buying a Barbie who happens to be named Ken.
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Sometimes, the male dolls aren't even named at all. Here are a couple of unnamed Barbie boy dolls, a groom and a prince (note: Mariposa isn't the prince's name, but the name of the line):
And finally, the doll I ultimately found to represent Heinrich Haussler isn't even a Ken. He's named Hudson, from the Barbie My Scene line. "Barbie Hudson" would mean absolutely nothing to most people out there, but I hope "Barbie Barbie" is immediately intelligible.
And, like I said, it sounds better.