Repeat each line using sed

Repeat each line using sed

I want to duplicate each line of the input using sed. I.e. when the input
looks like:
first
secod
third
fourth
...
I want the output to look like:
first first
second second
third third
fourth fourth
... ...
and so on.
If this is impossible using sed, then what do you recommend? I prefer the
most simple solution.