  Let ${\bf A}$ be the ring of adeles of a number field $F$. Given a self-dual
irreducible, automorphic, cuspidal representation $\tau$ of $\GL_n(\BA)$, with
trivial central characters, we construct its full inverse image under the weak
Langlands functorial lift from the appropriate split classical group $G$. We do
this by a new automorphic descent method, namely the double descent. This
method is derived from the recent generalized doubling integrals of Cai,
Friedberg, Ginzburg and Kaplan \cite{CFGK17}, which represent the standard
$L$-functions for $G\times \GL_n$. Our results are valid also for double covers
of symplectic groups.
