A real photo postcard is just a photograph that was developed on a thicker paper stock with a postcard back. If I had to guess about this one, I would think that maybe the printer ran out of regular photo paper and used a sheet of postcard paper instead. Or they might have done it accidentally. Either way, I personally wouldn't consider it a "double postcard." Just a printing (as in the stock it was printed on) anomaly.
-Ryan
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