wallet: ensure COutput added in set are unique

before #25806, set<COutput> was used and would not
contain same COutputs in the set.

now we use set<shared_ptr<COutput>> and it might be
possible for 2 distinct shared_ptr (different pointer
address but same COutputs) to be added into the set.

so preserve previous behaviour by making sure values
in the set are also distinct

src/util/insert.h

@@ -19,6 +19,11 @@ inline void insert(std::set<TsetT>& dst, const Tsrc& src) {
     dst.insert(src.begin(), src.end());
 }
 
+template <typename TsetT, typename Compare, typename Tsrc>
+inline void insert(std::set<TsetT, Compare>& dst, const Tsrc& src) {
+    dst.insert(src.begin(), src.end());
+}
+
 } // namespace util
 
 #endif // BITCOIN_UTIL_INSERT_H