A dead ringer is because people used to be buried alive. They'd break their graves and would walk around amongst the living, but people thought that that person was dead. Eventually, a system was put in place that those buried would have a ring, hence dead ringer.
Or, at least, that's what my English teacher said once.
Yes, grave sites were getting hard to find so they dug up the coffins and sent the bones to a bone house and reused the coffins. But they noticed scratch marks inside the lids and thought they were burying people alive so they ran a string up though the ground to a bell. There was a watchman in the cemetary who could hear the ringing and realize they had a "dead ringer" lol
I know, it's one of those stories that's so much more interesting than the truth (and a bit subversive to boot) that it gets passed around everywhere. I believed it myself for a while. But I've seen so many stories like this debunked that I basically check EVERYTHING on snopes.com now. It's almost more fun to just believe it
As for the second set of specious etymologies put forth by the e-mail about the disposal of urine, we note that while human urine has been used in a number of cultures throughout history to tan animal skins, such tanning was typically done by families or small bands of semi-related people to process the skins of animals they'd hunted or raised themselves. Folks weren't collecting their urine, then selling it to large commercial tanneries (which used other chemical compounds in their processing of animal skins).
Well I can't speak to the origins of the term "Piss Poor" but I can say that urine was used in the tanning process in large scale manufacturing operations. The following link [link](pg 326) takes you to a book published in 1885 on the leather industry. By the time this book was published alternative chemicals had come onto the market, but this had been a recent development (in the previous 20 years or so).
This link is also interesting and gives some insight into where these manufacturers would have gotten their urine. [link]. The text on pg 132 of this 1868 book states that one company who did not use urine to dye their yarns instead "carted" away the urine of their workers to other woollen mills.