I don't either.
All I know is this: I last saved and closed my document, everything looked correct and proper. no doubling of anything.
I opened the very same document 2 minutes later on a Mac running Snow Leopard, and when the document pages drew on the screen, there was a doubling of the same text boxes and color-filled shapes on half the pages in the document.
It's not something I would ever do myself, and if I did do it, I would remember doing it. It'd be hard to forget doubling items on half of the pages, especially if no master pages are employed in the document.
later today, I 'll file it as a bug
dcat