There's also a "Copy Favorites List" icon in Sentinel. The second one from the left, on the bottom row of shortcut icons, the one that resembles two pieces of paper (it's just to the left of the A|E (to rename a list). You'll get a pop-up menu box to designate a name for your "new" (copied) FL.
Once you've copied it, if there are systems you want to "trim out" (remove), you can do that in the main window, without going into the Favorites list editor. Just click on the list to expand it out, so that you can see the systems in the right side pane. You can then hold down the <Ctrl> key, and click to select those you want to remove. Then, with them still highlighted, either use the Delete key, or <Right> click, and select 'Remove' to delete