Although any item can be faked, there is little overt reason to suspect this item. It is a known overprint, known German airmail stamp and has an authentication indicia on the back. Most German fakes of that era are common stamps sadly overprinted with swastikas and overt nazi imagery. It looks real to me. Also WHO is selling it would be a factor (there are 2 or so sellers of fake nazi junk actively out there).
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