Choose "Close" in the "File" menu to remove a layout of a panel and close the panel unless there are still layouts loaded. If multiple layouts are loaded into the current panel, a dialog appears. This allows selecting one or many layouts for closing. "Close All" will close all panels.
Choose "Clone" from the "File" menu to duplicate a panel. A new panel will be created that is an exact copy of the current one. Both, the current and the new panel are views to the same layout. This way, only one copy of the layout is held in memory.
Choose "Pull In Other Layout" to combine other layouts already loaded into the current panel. Basically, KLayout allows viewsing a layout in multiple panels, either on its own in different configurations or together with other layouts. "Pull In Other Layout" allows configuration of a panel to show another layout which has been loaded into another panel. In that sense it's the reverse of closing one layout from a panel showing multiple layouts.