There are approximately thirty genuine two-headed or two-tailed coins that have been authenticated by ANACS, PCGS or NGC. There are only four known "Two Tailed" U.S. coins: three Quarters and one Dime. One of the Quarters recently sold for $80,000.
I have handled most of the world coins that were struck with either two obverse or two reverse dies. Two of these are Canadian Copper Cents that were struck with two reverse dies and authenticated by PCGS. I purchased these in a Canadian Mint Error Collection and they have since been featured in a front page Coin World article. Click here for more info on two-headed and two-tailed coins.
The coin featured here is a discovery coin. It is the first reported and authenticated two-headed English Victorian Shilling (Young Head). This type was struck by the Royal Mint from 1838 to 1887. It was authenticated by the Royal Mint in 1969, certified by NGC and also authenticated by several mint error experts. Since this Two Headed mint error is quite circulated it was most likely carried as a pocket piece and it is amazing that it survived all these years.