Yes, people regum stamps although I'm not sure if its as widespread as what it used to be. Regumming was done to make a stamp to MNH or take a used stamp, remove the cancel, and make it look mint. Through experience, you can id regumming but sometimes the job was well done and may even fool the experts. It lowers the value of the stamp so if buying an expensive stamp, its always best to send it in. Other faults may be identified also. Its kind of the same as a cleaned coin.
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Doug