Double Prints are actually quite rare and valuable. These cards are destroyed at the plate as they are very easy to detect, so for one to make it into circulation is very uncommon. You might have a thousand dollar plus card.
I must emphasize "might" as getting a sought after card with a miprint is more valuable than non-valuable, however it is still something you might want to get appraised.
Get it appraised at your local card shop, but do not sell it to them. They will try to rip you off it is valuable at MOST shops. I would hold onto it. It is guaranteed to go up in value as its worth is not based on the format.