I have just started using Vim for writing small scripts while doing CTFs so this will have things that I am learning with time.
duplicate a line:
Copy paste are simple:
Shift+CTRL+C - copy
Shift+CTRL+V - paste
v to get into visual mode - this is just for characters
V - this is for line mode
Ctrl+v - Visual block mode
Use arrow keys to select the text.
Shift+$ - to completely select the word.
Y - yank(copy)
d - delete
p - paste
u - undo
Break on comma
Show line number
Select the first caracter of your block
press Ctrl+V ( this is rectangular visual selection mode)
j for each line more you want to be commented
Shift-i (like I for "insert at start")
type // (or # or " or ...)
You will see the modification appearing only on the first line
IMPORTANT LAST STEP: type Esc key, and there you see the added character appear on all lines