Bilby has done something quite interesting. If one goes to the Weiner article, the sentence in question is (verbatim):
The impulse among musicians to create doubles — call them poppelgängers — is decades old
Why did bilby drop the 's'? I'm guessing it's because he knows, being the erudite marsupial that he is, that poppelgänger is already a plural form.
Which raises the question - was Weiner wrong to have written "poppelgängers"? How do we pluralize our foreign loanwords? Do we speak of our multiple doppelgänger or doppelgängers? Or maybe if we wanna be bad boiz, we could write doppelgängerz?
"The pop doppelgänger is not, of course, Beyoncé's innovation. Mariah Carey has Mimi, Eminem has Slim Shady, David Bowie had Ziggy Stardust, to name only a few. The impulse among musicians to create doubles—call them poppelgänger—is decades old, and it highlights one of pop music's basic contradictions: the way every performer, to some degree, becomes another person in order to express something true about himself."