Actually, there is only one copy of each topic. The Table of Contents is really just a list of links, like hyperlinks on a web page. The actual topic files are stored in Topic Files in the Project Files section of the Project Explorer. For convenience the topic files that TOC entries link to are displayed for editing automatically when you click on the entries.
You can always redock a window by double-clicking on its title bar (double-clicking also works for undocking, by the way). In addition to this you can dock or undock any dockable window by dragging it to the border of the main Editor window. Most windows can be docked to any side of the editor window.