scott schrieb:

>> What would likely happen is the system would detect the stamp was 
>> already used, and kick it back to the sender with "Insufficient 
>> Postage" emblazoned across the stamp.
> 
> I wonder if they even bother to check them against a database of already 
> used/issued ones?  Maybe they just check the checksum?

The German "Post" has a similar system, and they /do/ check it against a 
database.

BTW, the German 2D "stamp" barcode even encodes the full destination 
address information, which has the benefit (for the "Post") of (a) 
rendering the stamp fundamentally un-reusable anyway except for mails to 
the same addressee, and (b) making the address even easier machine-readable.

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