[Chad Fairbanks]

I think Dead Ringer comes from this:

The definition of ringer, from which this phrase comes, is "substituted racehorse."

Unscrupulous racehorse owners have a fast horse and a slow horse that are nearly identical in appearance. They run the slow horse until the betting odds reached the desired level, then they substitute the ringer, who can run much faster. Dead, in this case means abrupt or exact, like in dead stop, or dead shot.

No reliable source is available, however. :0)

[AnAmericanMother]

Well, you've heard the one about the fellow who showed up at a cathedral to answer the ad. about needed a bellringer.

He looked kinda goofy, but the priest let him go up the belltower, says, "Show me what you can do." The guy backs up to the wall, takes a running start, jumps up and rings one of the bells with his head. Sounds great. "Are you O.K.?" "Yeah, this is the way I always do it." "I don't believe it." Guy takes another run at the bell, misses, sails out of the window and down to the street below.

A crowd gathers. As the priest runs down the stairs and out into the street, the policeman comes up. He looks at the priest and says, "Did you know this man?"

Priest says, "Officer, I don't know his name, but his face rings a bell."

Part II:

Another guy shows up to answer the ad. He says he's the brother of the other guy and he's determined to retrieve the family honor. After much reluctance, the priest lets him try. Same stupid method, same result. Guy sails out the window and down to the square. A crowd gathers. Policeman asks the same question, and the priest replies,

. . . .. drum roll please . . . .

"I don't know his name, but he's a dead ringer for his brother."

bada bing.