How do I duplicate a whole line in Emacs?

I use

C-a C-SPACE C-n M-w C-y

which breaks down to

• C-a: move cursor to start of line
• C-SPACE: begin a selection (“set mark”)
• C-n: move cursor to next line
• M-w: copy region
• C-y: paste (“yank”)

The aforementioned

C-a C-k C-k C-y C-y

amounts to the same thing (TMTOWTDI)

• C-a: move cursor to start of line
• C-k: cut (“kill”) the line
• C-k: cut the newline
• C-y: paste (“yank”) (we’re back at square one)
• C-y: paste again (now we’ve got two copies of the line)

These are both embarrassingly verbose compared to C-d in your editor, but in Emacs there’s always a customization. C-d is bound to delete-char by default, so how about C-c C-d? Just add the following to your .emacs:

(global-set-key "\C-c\C-d" "\C-a\C- \C-n\M-w\C-y")

(@Nathan’s elisp version is probably preferable, because it won’t break if any of the key bindings are changed.)

Beware: some Emacs modes may reclaim C-c C-d to do something else.