- You cannot marshal locations or attachments as duplicates either.
You can marshal duplicates, because the card did not state otherwise. — Madnico 1
"Marshal a duplicate" is not a distinct player action. You initiate a "marshal a character/location/attachment" player action, and the card enters play as a duplicate. And while Marching Order is revealed, you cannot initiate the "marshal a location/attachment" player action. See the ruling on Arbor and duplicates, also the ruling on Barring the Gates. — mplain 176