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 poss

Answer 1

Try this simple one:

:g/^/norm yyp

Yet another one(shorter):

:%s/.*/&\r&

Another one:

:%!sed p