What is a "dead ringer"?

The reason it is ringer is explained here:

A ringer is a horse substituted for another of similar appearance in order to defraud the bookies. This word originated in the US horse-racing fraternity at the end of the 19th century. The word is defined for us in a copy of the Manitoba Free Press from October 1882:

"A horse that is taken through the country and trotted under a false name and pedigree is called a 'ringer.'"

The word dead is being used here in the same way it is used in :

Dead center/dead shot

From the same site:

So, that's ringer; what about dead? Dead, in the sense of lifeless, is so commonly used that we tend to ignore its other meanings. The meaning that's relevant here is exact or precise. This is demonstrated in many phrases; 'dead shot', 'dead centre', 'dead heat', etc.

This phrase then became used for anything that resembled another.

The dictionary gives its origin date:

Origin:
1890–95