and if I try to break down this problem into cases:
case 1: all distinct means I have {1,3,7}, and to make 2-digit distinct, I can possibly do permutation P(3,2)=6.
Case 2: two same digits :{3,3} , only one possibility =1
Total= 6+1=7;
Also, If you enumerate it like set with repeated elements={1,3,3,7}; and you ought to make 2-element ordered set then, they are : {1,3} ,{3,1},{1,7},{7,1},{3,7},{7,3},{3,3}=7.