It is called ghosting .The displacement of the metal when struck can show the design on either sign of the coin .Its normally caused by there being more metal on the raised design and the metal on the fields being thinner .Its simply the metal distorting when the coin is struck .Was common on the early George v pennies ,till they changed the design .I have seen quite a few of these on facebook forums ,so they probably are not a rarity .Its also possible that if its a design flaw ,that they all might have a bit of ghosting to some degree.