Paper was in short supply during the Civil War, and sometimes States/banks would reutilize and print on the reverse of other obsolete banknotes/checks/printings. I can't identify the source of the reverse, but perhaps someone else might.
The back was probably originally a bill of exchange. This is based on the reference to Duplicate and Original. They would have been sent by different methods. Whichever one was received first was cashed.
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