noun (n)

• a person or thing having the same function or characteristics as another(noun.cognition)

  Synonym:
  opposite number, vis-a-vis
  

  source: wordnet30
• a duplicate copy(noun.artifact)

  Synonym:
  similitude, twin
  

  source: wordnet30
• A part corresponding to another part; anything which answers, or corresponds, to another; a copy; a duplicate; a facsimile.(noun)

  source: webster1913