I'm not sure what caused this , I have one exactly like yours and I have never decided for sure what made them this way. the extra 8 , 7, and mintmark are too far apart to be what is called plating doubling in my opinion. so far the one you have and the one I have are the only two I have seen. it's possible this may be because the die was worn out and kept on striking these coins like this.
Honestly, I can't see a gosh-darn thing with the single photo you've provided. Chances are good that it is nothing more than strike doubling, considering there are no DDOs listed for that date/mintmark. -Brian
