To reproduce:
case #1 Dock a window to RadDock. Drag a window out of the Dock to make it floating. Check the FloatingWindowsCollection. Dock the window again, you will notice that the collection did not change.
case #2 Have a RadDock with a docked ToolWindow. Undock the ToolWindow by double-clicking it and dock it back. Now undock it by dragging it. If you now check the FloatingWindows collection you will see that there are two windows with the same name.

Resolution: 
The FloatingWindows property gets a list of the floating windows. Can contain duplicates, empty or hidden toolwindows which are used internally as re-dock targets. Use the ActiveFloatingWindows property instead of it.