No D/D Your Cent is a Copper plated Zinc Cent As stated that is Exposure to the plating and this would be te result of it if it continues..
By the way.. I just want to see if you are sure by what you mean by doubling. In your title you stated D/D.. In numismatic terms that mean a D over D Mint Mark or RPM - Repunched Mint Mark Since you stated doubling in your post I think you are asking if it is a Doubled Die which would be DDO Doubled Die Obverse or DDR Doubled Die Reverse. Just trying to help. I hope that anybody who reads this won't get offended
