dead ringer

A person or object that exactly resembles another, an exact counterpart in appearance. The usage of “ringer” for look-alike has been around since the late 1800s, when it was used for a horse that was fraudulently substituted for another in a race. It also was applied to the person who made such a substitution, but this usage has died out. However, in 1891 the term was made more emphatic with the addition of “dead,” here used in the sense of “exact,” as it is in DEAD HEAT for an exact tie.