1. (n.) One of two parts that complete each other\; anything which answers, or corresponds, to another; a copy; a duplicate; a facsimile.

2. (n.) One of two corresponding copies of an instrument; a duplicate.

3. (n.) A person who closely resembles another.

4. (n.) A thing may be applied to another thing so as to fit perfectly, as a seal to its impression; hence, a thing which is adapted to another thing, or which supplements it; that which serves to complete or complement anything; hence, a person or thing having qualities lacking in another; an opposite.

ally alter ego analogon analogue associate brother clone close copy close match coequal cognate companion compeer complement congenator congener coordinate correlate correlative correspondent ditto double dupe duplicate duplication each other equal equipollent equivalent facsimile fellow image kindred spirit like likeness match mate model near duplicate obverse one another opposite number parallel peer pendant picture quadruplicate reciprocal reciprocatist reciprocator repetition replica replication representation reproduction rival second self similitude simulacrum sister soul mate such suchlike tally the like of the likes of triplicate twin vis-a-vis