bug#46621: Copy line

[Juri Linkov]

>> In programming modes, if you need to declare a bunch of variables, you
>> may duplicate the current line several times and then proceed to rename
>> them.  I see the proposed new command may be useful in that scenario.
>
> Right; makes sense.
>
>> This has been a hot topic in Q&A sites:
>> https://stackoverflow.com/questions/88399/how-do-i-duplicate-a-whole-line-in-emacs
>
> OK, so there seems to be some general demand for this command, so I
> think we should go ahead and add it, unless somebody strongly objects.

Also tired of reading endless discussions on Q&A sites about reinventing
such fundamental feature as duplicating a line.

>> Also, why not name the command `duplicate-line'? I think it's a more
>> descriptive name and more discoverable via apropos (along with
>> `delete-duplicate-lines', for example).
>
> That does seem like an even better name for the command.  What do you
> think, Juri?

`duplicate-line' is a better name, but the problem is that is based on the
existing command `copy-from-above-command'.  Should they share
the same name prefix?  Please decide, I have no opinion on this. :-)