When printing of the the Penny Black began in 1840, the first printing plate started to wear quickly as it had not been hardened.  It was taken out of service and extensively repaired before printing re-commenced.  The unrepaired plate is called plate 1A and the repaired plate is 1B.  Several of the stamps from the repaired plate 1B show doubling of the design where the die did not line up exactly with the engraved design. 

 

The early Victorian stamps had different "check letters" in the bottom corners to combat forgery - the top left stamp in the sheet had AA checkletters, the next had AB, then AC and so on, and the next row was BA, BB etc.

 

One of the most pronounced doubling of the design occurred on stamp "MH" of plate 1B - in particular the "star" design in the top right corner is heavily doubled.

 

This makes this stamp particularly rare - only one of 240 stamps in sheets printed from the repaired plate 1 (there were 11 different plates in total produced, 12 if you count 1A and 1B separately).

 

A large British stamp dealer has a superb example of this stamp currently for sale at £485 - this stamp is not quite so fine, but a very nice example nevertheless.