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.