The original die was engraved with a double image so the coin will show this on certain devices. On the coin you mention, it is the designer's initials which are doubled.
Kind of hard to see in the pic from CoinsandCanada.com below, so I added some arrows to show some of the doubling:
So in this case double die obverse and double HP are saying the same thing. Makes sense now. The 1947 ML I own is a double HP and is ICCS graded and marked x2 HP. I had not heard of the DDO letters before so thanks for the info.
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