${\mathrm{K}}_2{\mathrm{SO}}_4.{\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3.24{\mathrm{H}}_2\mathrm{O}$ is 

 ${\mathrm{K}}_2{\mathrm{SO}}_4.{\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3.24{\mathrm{H}}_2\mathrm{O}$ is an example of double salt. It gives positive test for all the ions present in the formula.

Salt which on complete ionisation gives two different cations and a simple anion or a simple cation or two different simple anions is double salt