Empty Collation Folders stack in the writing desk allowing page duplication.

[ErdrickXLII]

Placing two or more empty Collation Folders into one 'tab' of the writing desk causes them to gain the same NBT data, causing the pages to duplicate. Further, if you have a stack of three or more Folders in the 'tab' and place a page in them, when you take the Folder out, all but two of the Folders are deleted (I would guess it's since the maximum stack size changed because they are no longer empty).