Right clicking on a Level map doesn’t show up any option for Duplicating it in 4.10. CTRL+W doesn’t work either. The option is visible and usable in 4.8. While there may be a work around (Copy the Level to another folder, rename it there and move it back to where it belongs).
Is there a specific reason why this feature has been taken out form 4.10?
As for the reason, I’m do not have any specifics other than that I see a couple of crash issues over the last couple of releases in relation to this feature. This was likely removed because it would cause more trouble than was needed.
You can easily just hold Ctrl + Shift + S to save as and name give the map a new name here.