My daughter came home one day with this wild tale a friend of hers told her about how, back in the middle ages, men would drink liquor out of cups that had some metal in them that would make them paralyzed if they drank too much. She went on to tell me how these people would be pronounced dead and then be buried, only to wake up buried alive. The story got more outrageous. So many of these folks were buried alive that, when buried, a hole would be dug in the ground leading to the casket. A string would be tied to the dead person's finger and the sting would lead to a bell on a stick above ground. That way, if the dead person "woke up," he could ring the bell.
I promised her it was all a lie. She didn't believe me, of course, since I'm the dumbest sonofabitch on earth right now. (When she was just one year younger, I was the smartest fellow on the planet. I tell you that just in case this happens to you. You can rest assured it happens to us all. Folks tell me that she'll be in her mid-20's before I have a grain of sense in my head again. I may not last that long. I hope you do.)
I promised her I would find proof of my argument on the internet. Well, I promptly forgot all about it (adding further credence to her theory about me being a dumbass). However, just now, in trying to find the origin of the term "dead ringer," there it is!
It turns out that one of the things in this internet hoax that was going around was the allegation that the term dead ringer comes from this very phenomenon. I think the internet hoax article was called "Life in the 1500s" or something like that.
So, where did this phrase come from? The accepted theory is that it has to do with the term ringer itself. This comes from the phrase "ring the changes," which means to ring all the bells in a bell-tower in varying sequences. Thus, it means to repeat something in a variety of ways. As slang, "ring the changes" came to mean substituting a bad or false thing for a good thing.
In the late 1800s, ringer came to be a term used for a horse with a great racing record being substituted for an unknown horse in a race, thus allowing those in the know to clean up on the betting. The dead in dead ringer is just another word for "positively" or "absolutely." So, since the ringer had to look like the thing it was meant to replace, "dead ringer" now is known as anyone or anything that is basically indistinguishable from someone or something else.