"Double headed" vector

Built using stacks:

documentclass{article}
defvecsign{mathchar"017E}
defdvecsign{smash{stackon[-1.95pt]{vecsign}{rotatebox{180}{$vecsign$}}}}
defdvec#1{defuseanchorwidth{T}stackon[-4.2pt]{#1}{,dvecsign}}
usepackage{stackengine}
stackMath
usepackage{graphicx}
begin{document}
$ vec c vec A  dvec c dvec A$
end{document}

There remains a slight possibility for overlap, when, for example, having $dvec d dvec b$. That issue can be remedied by using a little kern to make a narrower dvec, as follows. To recover the original, reset shrinkage to 0mu.

documentclass{article}
defshrinkage{2.1mu}
defvecsign{mathchar"017E}
defdvecsign{smash{stackon[-1.95pt]{mkern-shrinkagevecsign}{rotatebox{180}{$mkern-shrinkagevecsign$}}}}
defdvec#1{defuseanchorwidth{T}stackon[-4.2pt]{#1}{,dvecsign}}
usepackage{stackengine}
stackMath
usepackage{graphicx}
begin{document}
$ vec c vec A  dvec c dvec A$
$dvec d dvec b $
end{document}