diff --git a/src/neo/Ledger/TransactionVerificationContext.cs b/src/neo/Ledger/TransactionVerificationContext.cs
index a2bae45b0b..e291974137 100644
--- a/src/neo/Ledger/TransactionVerificationContext.cs
+++ b/src/neo/Ledger/TransactionVerificationContext.cs
@@ -13,8 +13,16 @@ public class TransactionVerificationContext
         /// </summary>
         private readonly Dictionary<UInt160, BigInteger> senderFee = new Dictionary<UInt160, BigInteger>();
 
+        /// <summary>
+        /// Store oracle responses
+        /// </summary>
+        private readonly Dictionary<ulong, UInt256> oracleResponses = new Dictionary<ulong, UInt256>();
+
         public void AddTransaction(Transaction tx)
         {
+            var oracle = tx.GetAttribute<OracleResponse>();
+            if (oracle != null) oracleResponses.Add(oracle.Id, tx.Hash);
+
             if (senderFee.TryGetValue(tx.Sender, out var value))
                 senderFee[tx.Sender] = value + tx.SystemFee + tx.NetworkFee;
             else
@@ -25,13 +33,23 @@ public bool CheckTransaction(Transaction tx, StoreView snapshot)
         {
             BigInteger balance = NativeContract.GAS.BalanceOf(snapshot, tx.Sender);
             senderFee.TryGetValue(tx.Sender, out var totalSenderFeeFromPool);
+
             BigInteger fee = tx.SystemFee + tx.NetworkFee + totalSenderFeeFromPool;
-            return balance >= fee;
+            if (balance < fee) return false;
+
+            var oracle = tx.GetAttribute<OracleResponse>();
+            if (oracle != null && oracleResponses.ContainsKey(oracle.Id))
+                return false;
+
+            return true;
         }
 
         public void RemoveTransaction(Transaction tx)
         {
             if ((senderFee[tx.Sender] -= tx.SystemFee + tx.NetworkFee) == 0) senderFee.Remove(tx.Sender);
+
+            var oracle = tx.GetAttribute<OracleResponse>();
+            if (oracle != null) oracleResponses.Remove(oracle.Id);
         }
     }
 }
diff --git a/src/neo/SmartContract/Native/Oracle/OracleContract.cs b/src/neo/SmartContract/Native/Oracle/OracleContract.cs
index 80217a2c37..68794c112e 100644
--- a/src/neo/SmartContract/Native/Oracle/OracleContract.cs
+++ b/src/neo/SmartContract/Native/Oracle/OracleContract.cs
@@ -147,7 +147,8 @@ protected override void PostPersist(ApplicationEngine engine)
 
                 //Remove the request from storage
                 StorageKey key = CreateStorageKey(Prefix_Request).Add(response.Id);
-                OracleRequest request = engine.Snapshot.Storages[key].GetInteroperable<OracleRequest>();
+                OracleRequest request = engine.Snapshot.Storages.TryGet(key)?.GetInteroperable<OracleRequest>();
+                if (request == null) continue;
                 engine.Snapshot.Storages.Delete(key);
 
                 //Remove the id from IdList
@@ -157,7 +158,7 @@ protected override void PostPersist(ApplicationEngine engine)
                 if (list.Count == 0) engine.Snapshot.Storages.Delete(key);
 
                 //Mint GAS for oracle nodes
-                nodes ??= NativeContract.Designate.GetDesignatedByRole(engine.Snapshot, Role.Oracle, engine.Snapshot.PersistingBlock.Index).Select(p => (Contract.CreateSignatureRedeemScript(p).ToScriptHash(), BigInteger.Zero)).ToArray();
+                nodes ??= Designate.GetDesignatedByRole(engine.Snapshot, Role.Oracle, engine.Snapshot.PersistingBlock.Index).Select(p => (Contract.CreateSignatureRedeemScript(p).ToScriptHash(), BigInteger.Zero)).ToArray();
                 if (nodes.Length > 0)
                 {
                     int index = (int)(response.Id % (ulong)nodes.Length);
diff --git a/tests/neo.UnitTests/Ledger/UT_TransactionVerificationContext.cs b/tests/neo.UnitTests/Ledger/UT_TransactionVerificationContext.cs
index a34a8cfb91..311cf7fd51 100644
--- a/tests/neo.UnitTests/Ledger/UT_TransactionVerificationContext.cs
+++ b/tests/neo.UnitTests/Ledger/UT_TransactionVerificationContext.cs
@@ -47,6 +47,29 @@ private Transaction CreateTransactionWithFee(long networkFee, long systemFee)
             return mock.Object;
         }
 
+        [TestMethod]
+        public void TestDuplicateOracle()
+        {
+            // Fake balance
+            var snapshot = Blockchain.Singleton.GetSnapshot();
+
+            ApplicationEngine engine = ApplicationEngine.Create(TriggerType.Application, null, snapshot, long.MaxValue);
+            BigInteger balance = NativeContract.GAS.BalanceOf(snapshot, UInt160.Zero);
+            NativeContract.GAS.Burn(engine, UInt160.Zero, balance);
+            NativeContract.GAS.Mint(engine, UInt160.Zero, 8);
+
+            // Test
+            TransactionVerificationContext verificationContext = new TransactionVerificationContext();
+            var tx = CreateTransactionWithFee(1, 2);
+            tx.Attributes = new TransactionAttribute[] { new OracleResponse() { Code = OracleResponseCode.ConsensusUnreachable, Id = 1, Result = new byte[0] } };
+            verificationContext.CheckTransaction(tx, snapshot).Should().BeTrue();
+            verificationContext.AddTransaction(tx);
+
+            tx = CreateTransactionWithFee(2, 1);
+            tx.Attributes = new TransactionAttribute[] { new OracleResponse() { Code = OracleResponseCode.ConsensusUnreachable, Id = 1, Result = new byte[0] } };
+            verificationContext.CheckTransaction(tx, snapshot).Should().BeFalse();
+        }
+
         [TestMethod]
         public void TestTransactionSenderFee()
         {