you can transform any set into multiset. Just like in adamant's example:

typedef tree<pair<int, int>, ...> ordered_set;

ordered_set X;
int cnt = 0;

X.insert({1, cnt}); cnt += 1;
X.insert({1, cnt}); cnt += 1;
X.insert({2, cnt}); cnt += 1;

cout << X.find_by_order(1)->first << endl; // 1
cout << X.order_of_key({2, 0}) << endl; // 2