A function to duplicate whatever the line under the cursor is. If you are a programmer this is something that you probably do a lot. However C-a C-k C-y C-y or C-a M-spc C-e M-w gets old very fast. Instead, put the following function in your .emacs:
(defun duplicate()
"Duplicate it the line we are on"
(interactive)
(let (
(beg (line-beginning-position))
(end (line-end-position)))
(copy-region-as-kill beg end)
(beginning-of-line)
(forward-line 1)
(yank)
(newline)
(forward-line -1)))
and bind it to any key-kombo you like (I used C-c C-j), and duplicating lines is suddenly just a key press away.