Is it possible to have a two head arrow as a decorator?

Question

I would like to have the analogue of \overrightarrow{x} but with a two headed arrow (as in \twoheadrightarrow, but above the variable). Is this possible?

Answer 1

\documentclass{article}
\usepackage{mathtools}
\def\Overrightarrow#1{\mathrlap{\overrightarrow{\phantom{#1\mkern11mu}}}%
  \overrightarrow{#1}}

\begin{document}
$\Overrightarrow{x}$
\end{document}

Answer 2

In the MWE below, the user command to create double-right-pointing arrow accents is \overdoublerightarrow. Its definition uses the LaTeX commands \overset and \DeclareMathSymbol and relies on the fact that the double-right arrow resides in slot "10 of the AMSa symbols. (The precise definitions may be found in the file amssymb.sty.) Finally, I use the command \textstyle because the default style of accent symbols is \scriptstyle, which looks much too skinny.

\documentclass{article}
\usepackage{amsmath,amssymb}
\DeclareMathSymbol{\Doublerightarrow}{\mathalpha}{AMSa}{"10}
\newcommand\overdoublerightarrow[1]{%
   \overset{\textstyle\Doublerightarrow}{#1}}
\begin{document}
$\overrightarrow{z}$ $\overrightarrow{A}$

$\overdoublerightarrow{z}$ $\overdoublerightarrow{A}$
\end{document}