It looks like the S took a hit at that point. It is not possible for a mintmark of 1971 to be classifies as a DD. The MM was hand punched until 1990. Sometimes they needed more than one strike to get a good impression. If the punch was not positioned correctly the result was known as a re-punched mintmark (RPM). NOT what your coin has by the way. Knowledge of the minting process and how dies were made would be an advantage to you. Also realising that there are thousands of ways for a coin to be damaged after it has left the mint as opposed to genuine varieties and errors.
