wallet: coin selection, add duplicated inputs checks

As no process should be able to trigger this error
using the regular transaction creation process, throw
a runtime_error if happens to tell users/devs to
report the bug if happens.

src/wallet/coinselection.h

@@ -10,6 +10,8 @@
 #include <policy/feerate.h>
 #include <primitives/transaction.h>
 #include <random.h>
+#include <util/system.h>
+#include <util/check.h>
 
 #include <optional>
 
@@ -302,6 +304,17 @@ private:
     /** Total weight of the selected inputs */
     int m_weight{0};
 
+    template<typename T>
+    void InsertInputs(const T& inputs)
+    {
+        // Store sum of combined input sets to check that the results have no shared UTXOs
+        const size_t expected_count = m_selected_inputs.size() + inputs.size();
+        util::insert(m_selected_inputs, inputs);
+        if (m_selected_inputs.size() != expected_count) {
+            throw std::runtime_error(STR_INTERNAL_BUG("Shared UTXOs among selection results"));
+        }
+    }
+
 public:
     explicit SelectionResult(const CAmount target, SelectionAlgorithm algo)
         : m_target(target), m_algo(algo) {}