Looks to me to be an overstrike-But I could be wrong Check this link in the glossary for some other good info- http://mikebyers.com/
It's a "squeeze job", not an error. Two shillings were pressed into the surface of this penny. A hammer or a vise will do the trick.
thanks guys for the feedback. I got this piece from a mixed coin bin, and wasn't sure what sort of error, or whether its really an error... just curious.