To delete a line: Ctrl–A to go to the beginning of the line, then Ctrl–K to delete it, and another time Ctrl–K to remove the empty line. (I do not use Xcode very often, but I’m used to that in Emacs and other text inputs with Emacs-like bindings, and it seems to work in Xcode too.)
And to duplicate a line: I don’t know of many programs that have a command for that, but usually I just use Copy+Paste – in Xcode it’s CUA-like: Ctrl+A to go to the beginning of the line, Shift+↓ to select it, Command+C to copy and Command+*V to paste twice (once overriding the line and once appending to it).
(from a person that types and edits text all the time, so often in different programs, and occasionally gets pissed at having to distract himself with a dumb widget while making a little correction in a text input, that he just cannot avoid remembering these sequences and habits)