ctrl + d = duplicate line
I've been using Notepad++ for a couple of years now, and whenever I needed to duplicate a line I would copy and paste it. This is a pain when you are doimg something like adding a new 301 redirect to a htaccess file. Typically I have the url to redirect copied to my clipboard, I open up the htaccess file and have to copy an existing entry, thus losing the url to redirect.
Behold ctrl + d! That's right, it's taken me two years to discover that ctrl + d duplicates the line in which the cursor is positioned. Much better; now I can copy a url, ctrl + d to duplicate an existing 301 redirect entry in htaccess and paste the url in.