You can achieve that with the following replacement:
:%s/^.*$/&,&/g ^ ^
^.*$ pattern will match the whole line, and
& is a backreference to the whole match. So, the replacement is inserting back the whole match, a comma, and again the whole match.
$ in this
^.*$ pattern can be omitted, but a lot of people prefer to keep them explicit in the pattern to facilitate pattern readability and further pattern testing.
An alternative in case you want to avoid matching empty lines:
.\+ will match 1 or more characters.