duplicate line in vim

vim

: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.