Dead Ringer
A dead ringer is something that is exactly duplicated. A horse that is substituted to fraud bookies is a 'ringer'. The word 'dead' in this idiom means 'exact'.

We recieved these emails from site visitors:
Email 1: I have always heard that a dead ringer is used because in England people who were afraid of being buried alive would attach a string to their finger that ran up to a bell, if they were alive they would ring the bell and be dug up. Making them a dead ringer.

Email 2: When playing horseshoes, a perfect throw resulting in a horseshoe encircling the scoring stake is called a "ringer". In the nineteenth century and on into the twentieth, the term "dead" could sometimes be used to mean "absolute" or "complete". A "dead ringer" occurs when a goal is achieved perfectly. The term comes from the game of horseshoes.

  (来源：英语论坛 http://bbs.englishcn.com)