Double face printings are rare. They are also quite stunning to see in person. You feel like you are drunk/high/ill when you see one. These are often times faked, so be careful if you are an interested buyer.
The double face printing error occurs when the second printing is reapplied on a sheet of currency that already has a second printing applied.
Double face printing errors are rare and valuable. We are very aggressive buyers. Please send us an image of your misprint so we can make an offer. firstname.lastname@example.org