There are some "mul" 20p's out there that may be worth some mney to a collector, Indeed the Royal mint is offering £50 each for such coins...
Therefore there are a lot of "people" trying to make a quick few quid out of the unsuspecting buyer, by being very carefull with descriptions etc. and selling a normal regular 20p for a lot more than face value.
the genuine "rare" 20ps are as follows:
1) they have No date on the HEAD side
2) they have a coat of arms type thing on the Tails side.
All 2008 coins with the coat of arms on the tails side, HAVE NO DATE ON THE TAILS SIDE, these are NORMAL 20ps and have a value of.. well.. 20p
A lot of older coins HAVE NO DATE ON THE HEAD SIDE (but do have a date on the tail side)