Choose Window > Arrange > "New Window for (document name)" and another window containing the same document will appear.
This is not a duplicate image (notice the file name displayed in the title bar is exactly the same). It is merely a second view of the very same image. Any edits you make in either window will appear in both windows.
This allows you to set two different magnifications for the same image simultaneously. You'll find this very useful when you need to zoom in close on part of a document for editing fine details, and at the same time you can see the results of your editing in actual size.
