Definitions of duplicate:

• noun:   a copy that corresponds to an original exactly

  Example: "He made a duplicate for the files"

• noun:   something additional of the same kind
• verb:   make or do or perform again
• verb:   make a duplicate or duplicates of

  Example: "Could you please duplicate this letter for me?"

• verb:   increase twofold
• verb:   duplicate or match
• adjective:   being two identical
• adjective:   identically copied from an original

  Example: "A duplicate key"