In the simplest way - at least for my perspective :)
yy
orY
to yank the current line, including the newline character at the end of the lineP
to paste before the cursor, orp
to paste after
More info https://vim.fandom.com/wiki/Copy,_cut_and_paste
u
undo last changeCtrl-r
redo changes which were undone (undo the undos)