It basically just gives them one extra use of it. Not too big a deal. Their access to loot is supposed to be bonkers in the dream world anyways.
If they use up their dream copy of the scroll, they retain the physical one, but they don't get a new dream copy every time they go back into the dreamlands.
But they do!
First, each time the PCs enter the Dreamlands, they manifest dream bodies that resemble their waking bodies. These lucid bodies have all the same spells prepared and all their usual gear, but spells and items used in the dream world are not expended in reality.
That is, PCs in the Dreamlands have their usual gear.
So as long as they don't use it in reality they keep getting a new one in the dream?