(This is a repeat of an old wish of mine. It's been a bit more than four years since I originally posted it so I hope it's acceptable to post it again. I thought things might have changed in the editor since then so perhaps it's fairly simple to implement it now.)
If I select a number of lines and use the Duplicate Line command, only the line with the cursor is duplicated.
I propose that if a selection exists, the full selection should be duplicated. If one line is selected (or none, the cursor is just placed somewhere), it should work as it currently does.