1964-D DD? Let Me Know What You Think.

Prior to 1990, the mint marks were punched into the die by hand after the die was made. When you see the same doubling on the coin's other devices as appears on the mint mark, it is a good sign that it is mechanical damage. If it were a doubled die, the mint mark would not show the same doubling. It is possible for a coin to exhibit both a doubled die and Mechanical Doubling.

To understand this better, you need to understand the die making process. The lower half of this web page explains how dies are mad: http://www.lincolncentsonline.com/doubledDies.html
If doubling occurs on any of the dies during this process, all of the coins minted with that die will show the same doubling. If the master die used for that year is doubled, all of the working hubs created with it will be doubled also.