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

Use the global command g to operate on every line in the file:

:g/^/norm yyp

The g command will operate on all lines that match a pattern. ^ is a pattern which will match any line. norm executes the command yyp, which yanks the current line, and pastes it. :g/^/norm Yp will also work.

See :help global for more details about the command, and see also this vim wiki page on g.