You just need to locate the command used by the key binding and place it wherever you want (Packages/User, usually).
Here's this particular one extracted directly from the repo:
code.google.com/p/sublime-text-c ... werUser.py
Now you only need to add a key binding for it and you're good to go.
Duplicates Current Line or Selected text
Duplicates current line if there's nothing selected. Else duplicates content
def run(self, view, args):
for region in view.sel():
line = view.line(region)
lineContents = view.substr(line) + '\n'
s = view.substr(region)