Double printed serial numbers and seals are very rare. There has to be a serious error during the printing process for such an error to occur. The example below had the third printing applied twice. The second printing has a different serial number.
These errors are easy to fake, but simple to identify, Be careful if you are looking to buy.
We are very interested in purchasing double printing errors. Please send us a picture of your double serial number error so that we can make an offer. firstname.lastname@example.org