You can copy items from a reading list either to the same item or another of your reading lists.
To copy an item click on the Copy icon (far right of item) and a popup window will appear showing all the reading lists you can copy the item to. Select one or more reading lists by clicking on the tick-box (to left of reading list names) and then click on the Copy link (bottom-left of popup window). The copied item will be appended to the bottom of the reading list(s) selected.