vi/vim editor, copy a block (not usual action)

just use V to select lines or v to select chars or Ctrlv to select a block.

When the selection spans the area you'd like to copy just hit y and use p to paste it anywhere you like...

Their Documentation says:

Cut and paste:

1. Position the cursor where you want to begin cutting.
2. Press v to select characters (or uppercase V to select whole lines).
3. Move the cursor to the end of what you want to cut.
4. Press d to cut (or y to copy).
5. Move to where you would like to paste.
6. Press P to paste before the cursor, or p to paste after.

Copy and paste is performed with the same steps except for step 4 where you would press y instead of d:

d = delete = cut

y = yank = copy

Another option which may be easier to remember would be to place marks on the two lines with ma and mb, then run :'a,'byank.

Many different ways to accomplish this task, just offering another.

I found the below command much more convenient. If you want to copy lines from 6 to 12 and paste from the current cursor position.

:6,12 co .

If you want to copy lines from 6 to 12 and paste from 100th line.

:6,12t100

Source: https://www.reddit.com/r/vim/comments/8i6vbd/efficient_ways_of_copying_few_lines/