I. adjective
made up of two very closely matched or identical aspects, elements, individuals, or parts
the twin threats of inflation and recession
Synonyms: double, dual, paired
Related Word: bifold, binary, twofold; identical, matched, matching; like, similar
Contrasted words: independent, separate; dissimilar, unlike
II. noun
Synonyms: mate 5, companion, coordinate, double, duplicate, fellow, match, reciprocal