Meaning an identical looking person, the term dead ringer originally referred to a horse that had been substituted for another of similar appearance to defraud the bookies. The verb to ring has long been used by the criminal fraternity to mean to substitute. The most modern example is to ring a car, meaning to steal another car's identity - usually for the purpose of reintroducing a write-off back on the road. Dead is widely used to mean accurate or precise, as in dead centre or dead heat.
A Competing Theory
Scratches found on the inside of coffins suggested that the occupants had erroneously been buried alive. To overcome this, a string would be tied to a finger of the person being buried and attached to a bell on the surface. If the bell rang, the person (who was obviously not dead) could be rescued. Of course, the resurfaced person would be identical to the person buried (it was the same person) and would be known as a dead ringer for the deceased.
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