Sorry to say but you have a damaged coin. It was sandwiched between two other coins and hammered, which mangled the rims and produced an incuse mirror image. A true double struck coin would have two identical overlapping strikes in relief.
A quick way to distinguish a "sandwich job" from a true double strike: the second strike will be more prominent and in relief. This coin shows the second "strike" as much softer and incuse on the relatively unaffected original strike.
Hey guys, thanks for the replies, I guess my only question now is why would someone bother to do this? Ah well, it's still my lucky coin anyway! And from reading up on error coins I now keep my eyes peeled for one! cheers