diff --git a/handshake_messages.go b/handshake_messages.go
index 17cf859..7ab0f10 100644
--- a/handshake_messages.go
+++ b/handshake_messages.go
@@ -384,6 +384,7 @@ func (m *clientHelloMsg) unmarshal(data []byte) bool {
 		return false
 	}
 
+	seenExts := make(map[uint16]bool)
 	for !extensions.Empty() {
 		var extension uint16
 		var extData cryptobyte.String
@@ -392,6 +393,11 @@ func (m *clientHelloMsg) unmarshal(data []byte) bool {
 			return false
 		}
 
+		if seenExts[extension] {
+			return false
+		}
+		seenExts[extension] = true
+
 		switch extension {
 		case extensionServerName:
 			// RFC 6066, Section 3
@@ -750,6 +756,7 @@ func (m *serverHelloMsg) unmarshal(data []byte) bool {
 		return false
 	}
 
+	seenExts := make(map[uint16]bool)
 	for !extensions.Empty() {
 		var extension uint16
 		var extData cryptobyte.String
@@ -758,6 +765,11 @@ func (m *serverHelloMsg) unmarshal(data []byte) bool {
 			return false
 		}
 
+		if seenExts[extension] {
+			return false
+		}
+		seenExts[extension] = true
+
 		switch extension {
 		case extensionStatusRequest:
 			m.ocspStapling = true
diff --git a/handshake_messages_test.go b/handshake_messages_test.go
index cc427bf..49452da 100644
--- a/handshake_messages_test.go
+++ b/handshake_messages_test.go
@@ -6,6 +6,7 @@ package tls
 
 import (
 	"bytes"
+	"encoding/hex"
 	"math/rand"
 	"reflect"
 	"strings"
@@ -463,3 +464,23 @@ func TestRejectEmptySCT(t *testing.T) {
 		t.Fatal("Unmarshaled ServerHello with zero-length SCT")
 	}
 }
+
+func TestRejectDuplicateExtensions(t *testing.T) {
+	clientHelloBytes, err := hex.DecodeString("010000440303000000000000000000000000000000000000000000000000000000000000000000000000001c0000000a000800000568656c6c6f0000000a000800000568656c6c6f")
+	if err != nil {
+		t.Fatalf("failed to decode test ClientHello: %s", err)
+	}
+	var clientHelloCopy clientHelloMsg
+	if clientHelloCopy.unmarshal(clientHelloBytes) {
+		t.Error("Unmarshaled ClientHello with duplicate extensions")
+	}
+
+	serverHelloBytes, err := hex.DecodeString("02000030030300000000000000000000000000000000000000000000000000000000000000000000000000080005000000050000")
+	if err != nil {
+		t.Fatalf("failed to decode test ServerHello: %s", err)
+	}
+	var serverHelloCopy serverHelloMsg
+	if serverHelloCopy.unmarshal(serverHelloBytes) {
+		t.Fatal("Unmarshaled ServerHello with duplicate extensions")
+	}
+}