Etymology of the word ‘Twin’
Old English twinn “consisting of two, twofold, double,” probably ultimately from Proto-Germanic *twinjaz (cf. Old Norse tvinnr, Old Danish tvinling, Dutch tweeling, German zwillung), from PIE *dwisno- (cf. Latin bini “two each,” Lithuanian dvynu “twins”), from *dwi- “double,” from root *dwo- “two” (see two). The verb meaning “to combine two things closely” is recorded from late 14c. The noun developed from Old English getwinn “double.”
Note the ultimate root of the word is the Proto-Indo-European ‘dwo-‘. which means ‘two’. whether it would have been used to describe a double or two of something remains unknown.
taken from – http://www.etymonline.com/index.php?term=twin