diff --git a/src/DBFTPlugin/Consensus/ConsensusService.OnMessage.cs b/src/DBFTPlugin/Consensus/ConsensusService.OnMessage.cs
index df358fdc4..cac84a8d4 100644
--- a/src/DBFTPlugin/Consensus/ConsensusService.OnMessage.cs
+++ b/src/DBFTPlugin/Consensus/ConsensusService.OnMessage.cs
@@ -127,13 +127,26 @@ private void OnPrepareRequestReceived(ExtensiblePayload payload, PrepareRequest
             {
                 if (mempoolVerified.TryGetValue(hash, out Transaction tx))
                 {
+                    if (NativeContract.Ledger.ContainsConflictHash(context.Snapshot, hash, tx.Signers.Select(s => s.Account)))
+                    {
+                        Log($"Invalid request: transaction has on-chain conflict", LogLevel.Warning);
+                        return;
+                    }
+
                     if (!AddTransaction(tx, false))
                         return;
                 }
                 else
                 {
                     if (neoSystem.MemPool.TryGetValue(hash, out tx))
+                    {
+                        if (NativeContract.Ledger.ContainsConflictHash(context.Snapshot, hash, tx.Signers.Select(s => s.Account)))
+                        {
+                            Log($"Invalid request: transaction has on-chain conflict", LogLevel.Warning);
+                            return;
+                        }
                         unverified.Add(tx);
+                    }
                 }
             }
             foreach (Transaction tx in unverified)
diff --git a/src/DBFTPlugin/Consensus/ConsensusService.cs b/src/DBFTPlugin/Consensus/ConsensusService.cs
index e50ab42d2..dc8f66ff1 100644
--- a/src/DBFTPlugin/Consensus/ConsensusService.cs
+++ b/src/DBFTPlugin/Consensus/ConsensusService.cs
@@ -257,7 +257,38 @@ private bool AddTransaction(Transaction tx, bool verify)
         {
             if (verify)
             {
-                VerifyResult result = tx.Verify(neoSystem.Settings, context.Snapshot, context.VerificationContext);
+                // At this step we're sure that there's no on-chain transaction that conflicts with
+                // the provided tx because of the previous Blockchain's OnReceive check. Thus, we only
+                // need to check that current context doesn't contain conflicting transactions.
+                VerifyResult result;
+
+                // Firstly, check whether tx has Conlicts attribute with the hash of one of the context's transactions.
+                foreach (var h in tx.GetAttributes<Conflicts>().Select(attr => attr.Hash))
+                {
+                    if (context.TransactionHashes.Contains(h))
+                    {
+                        result = VerifyResult.HasConflicts;
+                        Log($"Rejected tx: {tx.Hash}, {result}{Environment.NewLine}{tx.ToArray().ToHexString()}", LogLevel.Warning);
+                        RequestChangeView(ChangeViewReason.TxInvalid);
+                        return false;
+                    }
+                }
+                // After that, check whether context's transactions have Conflicts attribute with tx's hash.
+                foreach (var pooledTx in context.Transactions.Values)
+                {
+                    if (pooledTx.GetAttributes<Conflicts>().Select(attr => attr.Hash).Contains(tx.Hash))
+                    {
+                        result = VerifyResult.HasConflicts;
+                        Log($"Rejected tx: {tx.Hash}, {result}{Environment.NewLine}{tx.ToArray().ToHexString()}", LogLevel.Warning);
+                        RequestChangeView(ChangeViewReason.TxInvalid);
+                        return false;
+                    }
+                }
+
+                // We've ensured that there's no conlicting transactions in the context, thus, can safely provide an empty conflicting list
+                // for futher verification.
+                var conflictingTxs = new List<Transaction>();
+                result = tx.Verify(neoSystem.Settings, context.Snapshot, context.VerificationContext, conflictingTxs);
                 if (result != VerifyResult.Succeed)
                 {
                     Log($"Rejected tx: {tx.Hash}, {result}{Environment.NewLine}{tx.ToArray().ToHexString()}", LogLevel.Warning);