assume duplicate transactions are not added to iters_by_txid

In `AddTransactionsToBlock` description comment we have the asuumption
that callers will never pass multiple transactions with the same txid
We are asserting to assume that does not happen.

src/kernel/disconnected_transactions.cpp

@@ -51,7 +51,8 @@ size_t DisconnectedBlockTransactions::DynamicMemoryUsage() const
     iters_by_txid.reserve(iters_by_txid.size() + vtx.size());
     for (auto block_it = vtx.rbegin(); block_it != vtx.rend(); ++block_it) {
         auto it = queuedTx.insert(queuedTx.end(), *block_it);
-        iters_by_txid.emplace((*block_it)->GetHash(), it);
+        auto [_, inserted] = iters_by_txid.emplace((*block_it)->GetHash(), it);
+        assert(inserted); // callers may never pass multiple transactions with the same txid
         cachedInnerUsage += RecursiveDynamicUsage(**block_it);
     }
     return LimitMemoryUsage();