Harmy said:
I'm not sure what source was used for mastering the Aliens Blu-Ray but it is quite common to use an interpositive (a 2nd generation copy of the original negative) for home video masters. Dupe grain is the grain added by making a copy of a film strip by combining the grain already present in the source with the grain in the film strip onto which it is being copied.
Got it. Thanks Harmy.