Funny thing I just realized: russkies is a Russian-to-English borrowing with exactly the same kind of double plural as e.g. чипсы čipsy “potato crisps/chips”, an English-to-Russian one (a number of other Slavic languages do this as well, or so I’ve heard). That is, russkie is already a (nominative) plural in the source language (the nominative singular is russkij m, russkaâ f, russkoe n; it’s a nominalized adjective, thus the gendered forms). Presumably it doesn’t sound plural to the speaker of the target language, so(?) one more plural ending it gets. The only other Slavic-to-English example of this sort I know is pierogis, from Polish (the nominative singular form is pieróg). Any others?