$C{u}^{2+}\left(3d^9\right)$  is more stable than $C{u}^{+}\left(3d^{10}\right)$  in aqueous medium. This is due to

Greater charge and smaller size of $C{u}^{2+}$  ion than $C{u}^{+}$  ion

More extensive hydration of $C{u}^{2+}$  than $C{u}^{+}$  ion.

Free movement of one 3d electron over all the bonding orbitals in $C{u}^{2+}$  ion.

In aqueous solution $C{u}^{2+}$ is at stable electronic configuration