diff --git a/common.go b/common.go
index fdcebd8..1861efc 100644
--- a/common.go
+++ b/common.go
@@ -18,7 +18,6 @@ import (
 	"crypto/x509"
 	"errors"
 	"fmt"
-	"internal/godebug"
 	"io"
 	"net"
 	"strings"
@@ -977,9 +976,6 @@ var supportedVersions = []uint16{
 	VersionTLS10,
 }
 
-// debugEnableTLS10 enables TLS 1.0. See issue 45428.
-var debugEnableTLS10 = godebug.Get("tls10default") == "1"
-
 // roleClient and roleServer are meant to call supportedVersions and parents
 // with more readability at the callsite.
 const roleClient = true
@@ -991,7 +987,7 @@ func (c *Config) supportedVersions(isClient bool) []uint16 {
 		if needFIPS() && (v < fipsMinVersion(c) || v > fipsMaxVersion(c)) {
 			continue
 		}
-		if (c == nil || c.MinVersion == 0) && !debugEnableTLS10 &&
+		if (c == nil || c.MinVersion == 0) &&
 			isClient && v < VersionTLS12 {
 			continue
 		}
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 2f5d0e4..c6fc8f2 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")
+	}
+}
diff --git a/handshake_server_test.go b/handshake_server_test.go
index 16a2254..1f3a174 100644
--- a/handshake_server_test.go
+++ b/handshake_server_test.go
@@ -400,16 +400,6 @@ func TestVersion(t *testing.T) {
 	if err == nil {
 		t.Fatalf("expected failure to connect with TLS 1.0/1.1")
 	}
-
-	defer func(old bool) { debugEnableTLS10 = old }(debugEnableTLS10)
-	debugEnableTLS10 = true
-	_, _, err = testHandshake(t, clientConfig, serverConfig)
-	if err != nil {
-		t.Fatalf("handshake failed: %s", err)
-	}
-	if state.Version != VersionTLS11 {
-		t.Fatalf("incorrect version %x, should be %x", state.Version, VersionTLS11)
-	}
 }
 
 func TestCipherSuitePreference(t *testing.T) {