Some times we use (cout<<"/n" <<"/n" ; ) why do we use /n double time ?

Answer 1

\n equals a new line.

two \n equals two new lines.

It is easier to write cout << "\n\n"; than cout << " \n" << "\n" ; it is the same.

Answer 2

You could have stuff bw the lines breaks:
cout<<"something\nsomething else\nanotherthing";

Answer 3

We use \n two times to leave a blank line.