bug#46621: Copy line

[Drew Adams]

> > Do you feel audacious enough for a standard key binding?
> 
> Just to put it out there, if the result of the current discussion is
> that point is left at the beginning of the upper of the two lines, then
> using it would feel like using C-o, so duplicate-line could go on M-o.

Please don't give this command a default key binding.

There are a zillion commands that deserve a default
binding more than this one does.  And nearly all of
those zillion also don't need a default binding.