Go to the the scene folder in your windows explorer (or the equivalent explorer in mac) where your scene is saved. Copy the levels file that you want to duplicate and rename it as you want (the levels file is the .tlv file) Copy the .tpl file as well (the palette file) if you also want to copy the colors. Make sure the copied .tlv and .tpl files have the same file name. Go back to your x-sheet, right-click then click on 'load level'. Select the level you just copied. A pop out will appear saying "xxxxx doesn't belong to the current project...", select 'Load' on this pop out.
Since this is technically a different level from the one you copied it from, you can make edits on this new levels without affecting the original one.