diff --git a/include/openthread-types.h b/include/openthread-types.h
index 9422a65ab..757b2600d 100644
--- a/include/openthread-types.h
+++ b/include/openthread-types.h
@@ -167,6 +167,10 @@ typedef enum ThreadError
      */
     kThreadError_ResponseTimeout = 30,
 
+    /**
+     * Received a duplicated frame.
+     */
+    kThreadError_Duplicated = 31,
     kThreadError_Error = 255,
 } ThreadError;
 
@@ -822,6 +826,7 @@ typedef struct otMacCounters
     uint32_t mRxOther;                ///< The number of received other types of frames.
     uint32_t mRxWhitelistFiltered;    ///< The number of received packets filtered by whitelist.
     uint32_t mRxDestAddrFiltered;     ///< The number of received packets filtered by destination check.
+    uint32_t mRxDuplicated;           ///< The number of received duplicated packets.
     uint32_t mRxErrNoFrame;           ///< The number of received packets that do not contain contents.
     uint32_t mRxErrUnknownNeighbor;   ///< The number of received packets from unknown neighbor.
     uint32_t mRxErrInvalidSrcAddr;    ///< The number of received packets whose source address is invalid.
diff --git a/src/cli/README.md b/src/cli/README.md
index 016d54ea7..255c49714 100644
--- a/src/cli/README.md
+++ b/src/cli/README.md
@@ -360,6 +360,7 @@ RxTotal: 11
     RxOther: 0
     RxWhitelistFiltered: 0
     RxDestAddrFiltered: 0
+    RxDuplicated: 0
     RxErrNoFrame: 0
     RxErrNoUnknownNeighbor: 0
     RxErrInvalidSrcAddr: 0
diff --git a/src/cli/cli.cpp b/src/cli/cli.cpp
index 05a15c9e8..952fd2e82 100644
--- a/src/cli/cli.cpp
+++ b/src/cli/cli.cpp
@@ -512,6 +512,7 @@ void Interpreter::ProcessCounters(int argc, char *argv[])
             sServer->OutputFormat("    RxOther: %d\r\n", counters->mRxOther);
             sServer->OutputFormat("    RxWhitelistFiltered: %d\r\n", counters->mRxWhitelistFiltered);
             sServer->OutputFormat("    RxDestAddrFiltered: %d\r\n", counters->mRxDestAddrFiltered);
+            sServer->OutputFormat("    RxDuplicated: %d\r\n", counters->mRxDuplicated);
             sServer->OutputFormat("    RxErrNoFrame: %d\r\n", counters->mRxErrNoFrame);
             sServer->OutputFormat("    RxErrNoUnknownNeighbor: %d\r\n", counters->mRxErrUnknownNeighbor);
             sServer->OutputFormat("    RxErrInvalidSrcAddr: %d\r\n", counters->mRxErrInvalidSrcAddr);
diff --git a/src/core/mac/mac.cpp b/src/core/mac/mac.cpp
index a1a51916c..5a2939728 100644
--- a/src/core/mac/mac.cpp
+++ b/src/core/mac/mac.cpp
@@ -1059,10 +1059,22 @@ ThreadError Mac::ProcessReceiveSecurity(Frame &aFrame, const Address &aSrcAddr,
             ExitNow(error = kThreadError_Security);
         }
 
-        VerifyOrExit((keySequence > aNeighbor->mKeySequence) ||
-                     ((keySequence == aNeighbor->mKeySequence) &&
-                      (frameCounter >= aNeighbor->mValid.mLinkFrameCounter)),
-                     error = kThreadError_Security);
+        if (keySequence < aNeighbor->mKeySequence)
+        {
+            ExitNow(error = kThreadError_Security);
+        }
+        else if (keySequence == aNeighbor->mKeySequence)
+        {
+            if ((frameCounter + 1) < aNeighbor->mValid.mLinkFrameCounter)
+            {
+                ExitNow(error = kThreadError_Security);
+            }
+            else if ((frameCounter + 1) == aNeighbor->mValid.mLinkFrameCounter)
+            {
+                // drop duplicated packets
+                ExitNow(error = kThreadError_Duplicated);
+            }
+        }
 
         extAddress = &aSrcAddr.mExtAddress;
 
@@ -1109,11 +1121,11 @@ ThreadError Mac::ProcessReceiveSecurity(Frame &aFrame, const Address &aSrcAddr,
 
 exit:
 
-    if (error != kThreadError_None)
+    if (error == kThreadError_Security)
     {
         for (Receiver *receiver = mReceiveHead; receiver; receiver = receiver->mNext)
         {
-            receiver->HandleReceivedFrame(aFrame, kThreadError_Security);
+            receiver->HandleReceivedFrame(aFrame, error);
         }
     }
 
@@ -1322,6 +1334,10 @@ exit:
             mCounters.mRxDestAddrFiltered++;
             break;
 
+        case kThreadError_Duplicated:
+            mCounters.mRxDuplicated++;
+            break;
+
         default:
             mCounters.mRxErrOther++;
             break;
diff --git a/src/ncp/ncp_base.cpp b/src/ncp/ncp_base.cpp
index dfdc5b0bf..54097df16 100644
--- a/src/ncp/ncp_base.cpp
+++ b/src/ncp/ncp_base.cpp
@@ -179,6 +179,7 @@ const NcpBase::GetPropertyHandlerEntry NcpBase::mGetPropertyHandlerTable[] =
     { SPINEL_PROP_CNTR_RX_ERR_SECURITY, &NcpBase::GetPropertyHandler_MAC_CNTR },
     { SPINEL_PROP_CNTR_RX_ERR_BAD_FCS, &NcpBase::GetPropertyHandler_MAC_CNTR },
     { SPINEL_PROP_CNTR_RX_ERR_OTHER, &NcpBase::GetPropertyHandler_MAC_CNTR },
+    { SPINEL_PROP_CNTR_RX_PKT_DUP, &NcpBase::GetPropertyHandler_MAC_CNTR },
 
     { SPINEL_PROP_CNTR_TX_IP_SEC_TOTAL, &NcpBase::GetPropertyHandler_NCP_CNTR },
     { SPINEL_PROP_CNTR_TX_IP_INSEC_TOTAL, &NcpBase::GetPropertyHandler_NCP_CNTR },
@@ -2438,6 +2439,10 @@ ThreadError NcpBase::GetPropertyHandler_MAC_CNTR(uint8_t header, spinel_prop_key
         value = macCounters->mRxDestAddrFiltered;
         break;
 
+    case SPINEL_PROP_CNTR_RX_PKT_DUP:
+        value = macCounters->mRxDuplicated;
+        break;
+
     case SPINEL_PROP_CNTR_RX_ERR_EMPTY:
         value = macCounters->mRxErrNoFrame;
         break;
diff --git a/src/ncp/spinel.h b/src/ncp/spinel.h
index e8c7ba6c7..5eaff6bad 100644
--- a/src/ncp/spinel.h
+++ b/src/ncp/spinel.h
@@ -728,6 +728,10 @@ typedef enum
     /** Format: `L` (Read-only) */
     SPINEL_PROP_CNTR_RX_ERR_OTHER      = SPINEL_PROP_CNTR__BEGIN + 113,
 
+    /// The number of received duplicated.
+    /** Format: `L` (Read-only) */
+    SPINEL_PROP_CNTR_RX_PKT_DUP        = SPINEL_PROP_CNTR__BEGIN + 114,
+
     /// The total number of secure transmitted IP messages.
     /** Format: `L` (Read-only) */
     SPINEL_PROP_CNTR_TX_IP_SEC_TOTAL   = SPINEL_PROP_CNTR__BEGIN + 200,