An example of double salt is 

Double salts are addition or molecular compounds which are formed by two apparently saturated compounds but they lose pair identity when dissolved in water. The most common example of double salt is potash alum ${\mathrm{K}}_2{\mathrm{SO}}_4.{\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3.24{\mathrm{H}}_2\mathrm{O}$.