Duplicate each line in VI

Question

I have a file with these lines:

aa
bb
cc
dd

I want to convert this into:

aa
aa
bb
bb
cc
cc
dd
dd

Is this possible in VI?

Answer 1

Try this simple one:

:g/^/norm yyp

Yet another one(shorter):

:%s/.*/&\r&

Another one:

:%!sed p

Answer 2

I like g/^/t.
The g (for global) command will look for any lines that match a pattern.
The pattern we specified is ^, which will match all lines.
t will copy and paste, and finally
the dot tells it to paste below.

Do I win for brevity?