How to write the arrow of the two overlap style?

That's some strange symbol...

documentclass{article}

usepackage{latexsym}
newcommand*{negdblleadsto}{leadstomkern-15munotleadsto}

begin{document}

$A negdblleadsto B$

end{document}

Note that latexsym is a relatively old package; however the more common amssymb version looks quite different. You might want to use amssymb but import the latexsym definition of leadsto as shown here.