How can I repeat the line in notepad++?

Question

How can I repeat the line in notepad++ ?

For example I have the following input:

a 01
a 02
a 03
a 04

And I would like it to become:

Answer 1

If you don't mind the semi-manual process, you can start at the first line and repeat the following key combination until you reach the end of the document (you get very fast at this)

Ctrl + D Down Down

This duplicates the current line, then moves down twice (to the line following the newly duplicated one).

If you have a large document, you can record this once as a macro (see the Macro menu), then use either Ctrl + Shift + P (Playback) repeatedly -- you can just hold those keys down. Or use the Macro option "Run a Macro Multiple Times..."