How to Insert a Page Break and Duplicate a Page in Pages 5.1
Pages 5 deprecated the "Word Processing" and "Page Layout" styles of document creation, but in 5.1, they have returned.
It seems that the DEFAULT is to "Word Processing," however it is possible to convert the document to a "Page Layout" document: In your open document, click File, then Convert To Page Layout.
So, for a Word Processing document, one only needs to click Insert and then Page Break, while in a Page Layout document, click Insert and then Page.
Unfortunately, the Duplicate Page function has NOT returned, but you can still select the entire contents of a page and copy it to a new page. Unfortunately, your perfect page layout won't copy over exactly, and guides don't copy over either.
Mac mini (Late 2012), OS X Mavericks (10.9.1)