How to duplicate a whole line in Vim?

YP or Yp or yyp.

Normal mode: see other answers.

The Ex way:

• :t. will duplicate the line,
• :t 7 will copy it after line 7,
• :,+t0 will copy current and next line at the beginning of the file (,+ is a synonym for the range .,.+1),
• :1,t$ will copy lines from beginning till cursor position to the end (1, is a synonym for the range 1,.).

If you need to move instead of copying, use :m instead of :t.

This can be really powerful if you combine it with :g or :v:

• :v/foo/m$ will move all lines not matching the pattern “foo” to the end of the file.
• :+,$g/^\s*class\s\+\i\+/t. will copy all subsequent lines of the form class xxx right after the cursor.

Reference: :help range, :help :t, :help :g, :help :m and :help :v

yy or Y to copy the line (mnemonic: yank)
or
dd to delete the line (Vim copies what you deleted into a clipboard-like "register", like a cut operation)

then

p to paste the copied or deleted text after the current line
or
P to paste the copied or deleted text before the current line