wallet: add assert to SelectionResult::Merge for safety

src/wallet/coinselection.cpp

@@ -452,12 +452,16 @@ void SelectionResult::AddInputs(const std::set<COutput>& inputs, bool subtract_f
 
 void SelectionResult::Merge(const SelectionResult& other)
 {
+    // Obtain the expected selected inputs count after the merge (for now, duplicates are not allowed)
+    const size_t expected_count = m_selected_inputs.size() + other.m_selected_inputs.size();
+
     m_target += other.m_target;
     m_use_effective |= other.m_use_effective;
     if (m_algo == SelectionAlgorithm::MANUAL) {
         m_algo = other.m_algo;
     }
     util::insert(m_selected_inputs, other.m_selected_inputs);
+    assert(m_selected_inputs.size() == expected_count);
 }
 
 const std::set<COutput>& SelectionResult::GetInputSet() const

src/wallet/coinselection.h

@@ -316,6 +316,12 @@ public:
     void ComputeAndSetWaste(const CAmount min_viable_change, const CAmount change_cost, const CAmount change_fee);
     [[nodiscard]] CAmount GetWaste() const;
 
+    /**
+     * Combines the @param[in] other selection result into 'this' selection result.
+     *
+     * Important note:
+     * There must be no shared 'COutput' among the two selection results being combined.
+     */
     void Merge(const SelectionResult& other);
 
     /** Get m_selected_inputs */