Every release can have it's mistakes rolling of the factory.
Two times the same poster? Might be a one-off indeed but is never a rarity. As anyone with two sets can 'make it'.
A misplaced label could also be a one-off, but is rarely considered a rarity as in that collectors have a special interest in them.
A misprint ON the label, on the other hand, COULD make it collectable.
Misprints that actually contain different music are a different story. They ARE in a lot of cases of interest for some collectors.
Think about the Dutch WWRY re-releases from a couple of years ago that played Magic instead of Friends will be friends.
But there are loads of these, most of them not known in general because they are probably the only copies around and the amount of collectors that are REALY interested in these is minimal.
A couple examples from my own collection:
Live At Wembley cd playing Dutch comedian Toon Hermans, The Cross Shove It! 12" single playing Belinda Carlisle...
Or the other way round: David Bowie Hunky Dory remaster cd playing the A Kind of Magic album.
Misprints that ARE of real interest and worth a lot of money in most cases are unique examples of albums that were pressed on the 'wrong' colour vinyl, sleeves that are in some way different.