COT, DD is on the die, while MD is a striking issue. No reason MD can't occur on a doubled die. While it adds a lot of pizzazz to the coin, generally this is not considered desirable.
ccobb, bulk rolls can be had at coin dealers, coin shows, eBay, etc., or just by asking an eBay seller with one roll up if they have a stash of them and want to sell bulk. This last set was 26 rolls I bought for $45. I didn't even know what dates were in there or what condition I might find. Been through three of them so far, with the 1970-S scoring nicely on good coins, the 1963-D had nothing and all were so spotty I just tossed them in the "to the bank" bag, and the third I just finished was a 1961-D with great coins and some really good RPMs. I'll do a post of the RPMs like I did with these 1970-S DDOs, but I have to get them all labeled up and all first. There are 14 RPM-007 coins alone so takes a while to organize it all after going through a good roll.