In aqueous solutions, $Cu^{2+}$ is more stable than $Cu^+$. This is because the hydration enthalpy ($\Delta_{hyd}H^\circ$) of the $Cu^{2+}$ ion is much more negative than that of the $Cu^+$ ion. This highly negative hydration energy more than compensates for the high second ionisation enthalpy required to convert $Cu^+$ to $Cu^{2+}$ .