diff --git a/include/openssl/ssl.h b/include/openssl/ssl.h
index 36d17dd224f21bc6effb4ad66bc6551f759af3af..9709103d5fb984dda3003a7e0cd77460b1ceb2d6 100644
--- a/include/openssl/ssl.h
+++ b/include/openssl/ssl.h
@@ -2123,6 +2123,7 @@ void ERR_load_SSL_strings(void);
 # define SSL_F_STATE_MACHINE                              353
 # define SSL_F_TLS12_CHECK_PEER_SIGALG                    333
 # define SSL_F_TLS1_CHANGE_CIPHER_STATE                   209
+# define SSL_F_TLS1_CHECK_DUPLICATE_EXTENSIONS            341
 # define SSL_F_TLS1_CHECK_SERVERHELLO_TLSEXT              274
 # define SSL_F_TLS1_EXPORT_KEYING_MATERIAL                314
 # define SSL_F_TLS1_GET_CURVELIST                         338
diff --git a/ssl/ssl_err.c b/ssl/ssl_err.c
index e6b9bbdb9cc813c7e9cfe447cdd8d441088aaece..46f483febe29b1270efcb248761fd4253c9f3581 100644
--- a/ssl/ssl_err.c
+++ b/ssl/ssl_err.c
@@ -273,6 +273,8 @@ static ERR_STRING_DATA SSL_str_functs[] = {
     {ERR_FUNC(SSL_F_STATE_MACHINE), "state_machine"},
     {ERR_FUNC(SSL_F_TLS12_CHECK_PEER_SIGALG), "tls12_check_peer_sigalg"},
     {ERR_FUNC(SSL_F_TLS1_CHANGE_CIPHER_STATE), "tls1_change_cipher_state"},
+    {ERR_FUNC(SSL_F_TLS1_CHECK_DUPLICATE_EXTENSIONS),
+     "tls1_check_duplicate_extensions"},
     {ERR_FUNC(SSL_F_TLS1_CHECK_SERVERHELLO_TLSEXT),
      "TLS1_CHECK_SERVERHELLO_TLSEXT"},
     {ERR_FUNC(SSL_F_TLS1_EXPORT_KEYING_MATERIAL),
diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index 7a2047dcca1e5c7d676fc7b9238f2ff5fda79d57..db5f0f6b442a86b535a79276fccf491a8df8e410 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -109,6 +109,7 @@
  */
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <openssl/objects.h>
 #include <openssl/evp.h>
 #include <openssl/hmac.h>
@@ -124,7 +125,7 @@ static int tls_decrypt_ticket(SSL *s, const unsigned char *tick, int ticklen,
                               const unsigned char *sess_id, int sesslen,
                               SSL_SESSION **psess);
 static int ssl_check_clienthello_tlsext_early(SSL *s);
-int ssl_check_serverhello_tlsext(SSL *s);
+static int ssl_check_serverhello_tlsext(SSL *s);
 
 SSL3_ENC_METHOD const TLSv1_enc_data = {
     tls1_enc,
@@ -1037,6 +1038,79 @@ static int tls_use_ticket(SSL *s)
     return ssl_security(s, SSL_SECOP_TICKET, 0, 0, NULL);
 }
 
+static int compare_uint(const void *p1, const void *p2) {
+    unsigned int u1 = *((const unsigned int *)p1);
+    unsigned int u2 = *((const unsigned int *)p2);
+    if (u1 < u2)
+        return -1;
+    else if (u1 > u2)
+        return 1;
+    else
+        return 0;
+}
+
+/*
+ * Per http://tools.ietf.org/html/rfc5246#section-7.4.1.4, there may not be
+ * more than one extension of the same type in a ClientHello or ServerHello.
+ * This function does an initial scan over the extensions block to filter those
+ * out. It returns 1 if all extensions are unique, and 0 if the extensions
+ * contain duplicates, could not be successfully parsed, or an internal error
+ * occurred.
+ */
+static int tls1_check_duplicate_extensions(const PACKET *packet) {
+    PACKET extensions = *packet;
+    size_t num_extensions = 0, i = 0;
+    unsigned int *extension_types = NULL;
+    int ret = 0;
+
+    /* First pass: count the extensions. */
+    while (PACKET_remaining(&extensions) > 0) {
+        unsigned int type;
+        PACKET extension;
+        if (!PACKET_get_net_2(&extensions, &type) ||
+            !PACKET_get_length_prefixed_2(&extensions, &extension)) {
+            goto done;
+        }
+        num_extensions++;
+    }
+
+    if (num_extensions <= 1)
+        return 1;
+
+    extension_types = OPENSSL_malloc(sizeof(unsigned int) * num_extensions);
+    if (extension_types == NULL) {
+        SSLerr(SSL_F_TLS1_CHECK_DUPLICATE_EXTENSIONS, ERR_R_MALLOC_FAILURE);
+        goto done;
+    }
+
+    /* Second pass: gather the extension types. */
+    extensions = *packet;
+    for (i = 0; i < num_extensions; i++) {
+        PACKET extension;
+        if (!PACKET_get_net_2(&extensions, &extension_types[i]) ||
+            !PACKET_get_length_prefixed_2(&extensions, &extension)) {
+            /* This should not happen. */
+            SSLerr(SSL_F_TLS1_CHECK_DUPLICATE_EXTENSIONS, ERR_R_INTERNAL_ERROR);
+            goto done;
+        }
+    }
+
+    if (PACKET_remaining(&extensions) != 0) {
+        SSLerr(SSL_F_TLS1_CHECK_DUPLICATE_EXTENSIONS, ERR_R_INTERNAL_ERROR);
+        goto done;
+    }
+    /* Sort the extensions and make sure there are no duplicates. */
+    qsort(extension_types, num_extensions, sizeof(unsigned int), compare_uint);
+    for (i = 1; i < num_extensions; i++) {
+        if (extension_types[i - 1] == extension_types[i])
+            goto done;
+    }
+    ret = 1;
+ done:
+    OPENSSL_free(extension_types);
+    return ret;
+}
+
 unsigned char *ssl_add_clienthello_tlsext(SSL *s, unsigned char *buf,
                                           unsigned char *limit, int *al)
 {
@@ -1837,6 +1911,9 @@ static int ssl_scan_clienthello_tlsext(SSL *s, PACKET *pkt, int *al)
     if (PACKET_remaining(pkt) != len)
         goto err;
 
+    if (!tls1_check_duplicate_extensions(pkt))
+        goto err;
+
     while (PACKET_get_net_2(pkt, &type) && PACKET_get_net_2(pkt, &size)) {
         PACKET subpkt;
 
@@ -2301,6 +2378,11 @@ static int ssl_scan_serverhello_tlsext(SSL *s, PACKET *pkt, int *al)
         return 0;
     }
 
+    if (!tls1_check_duplicate_extensions(pkt)) {
+        *al = SSL_AD_DECODE_ERROR;
+        return 0;
+    }
+
     while (PACKET_get_net_2(pkt, &type) && PACKET_get_net_2(pkt, &size)) {
         const unsigned char *data;
         PACKET spkt;
diff --git a/test/recipes/70-test_sslcertstatus.t b/test/recipes/70-test_sslcertstatus.t
index 9a0c5f8eac406535b350c0f808eba6e6c861e299..303de5e56635cfec038868c7f711c24766723862 100755
--- a/test/recipes/70-test_sslcertstatus.t
+++ b/test/recipes/70-test_sslcertstatus.t
@@ -99,7 +99,7 @@ sub certstatus_filter
         if ($message->mt == TLSProxy::Message::MT_SERVER_HELLO) {
             #Add the status_request to the ServerHello even though we are not
             #going to send a CertificateStatus message
-            $message->set_extension(TLSProxy::ClientHello::EXT_STATUS_REQUEST,
+            $message->set_extension(TLSProxy::Message::EXT_STATUS_REQUEST,
                                     "");
 
             $message->repack();
diff --git a/test/recipes/70-test_sslextension.t b/test/recipes/70-test_sslextension.t
index 4582c5c72087aa13815812192b9336f90906a5c7..c253f748e0d84dc4d50877336ec19a53f83fc2c6 100755
--- a/test/recipes/70-test_sslextension.t
+++ b/test/recipes/70-test_sslextension.t
@@ -78,9 +78,9 @@ my $proxy = TLSProxy::Proxy->new(
     (!$ENV{HARNESS_ACTIVE} || $ENV{HARNESS_VERBOSE})
 );
 
-plan tests => 1;
+plan tests => 3;
 
-#Test 1: Sending a zero length extension block should pass
+# Test 1: Sending a zero length extension block should pass
 $proxy->start();
 ok(TLSProxy::Message->success, "Zero extension length test");
 
@@ -95,13 +95,64 @@ sub extension_filter
 
     foreach my $message (@{$proxy->message_list}) {
         if ($message->mt == TLSProxy::Message::MT_CLIENT_HELLO) {
-            #Remove all extensions and set the extension len to zero
+            # Remove all extensions and set the extension len to zero
             $message->extension_data({});
             $message->extensions_len(0);
-            #Extensions have been removed so make sure we don't try to use them
+            # Extensions have been removed so make sure we don't try to use them
             $message->process_extensions();
 
             $message->repack();
         }
     }
 }
+
+# Test 2-3: Sending a duplicate extension should fail.
+sub inject_duplicate_extension
+{
+  my ($proxy, $message_type) = @_;
+
+    foreach my $message (@{$proxy->message_list}) {
+        if ($message->mt == $message_type) {
+          my %extensions = %{$message->extension_data};
+            # Add a duplicate (unknown) extension.
+            $message->set_extension(TLSProxy::Message::EXT_DUPLICATE_EXTENSION, "");
+            $message->set_extension(TLSProxy::Message::EXT_DUPLICATE_EXTENSION, "");
+            $message->repack();
+        }
+    }
+}
+
+sub inject_duplicate_extension_clienthello
+{
+    my $proxy = shift;
+
+    # We're only interested in the initial ClientHello
+    if ($proxy->flight != 0) {
+        return;
+    }
+
+    inject_duplicate_extension($proxy, TLSProxy::Message::MT_CLIENT_HELLO);
+}
+
+sub inject_duplicate_extension_serverhello
+{
+    my $proxy = shift;
+
+    # We're only interested in the initial ServerHello
+    if ($proxy->flight != 1) {
+        return;
+    }
+
+    inject_duplicate_extension($proxy, TLSProxy::Message::MT_SERVER_HELLO);
+}
+
+$proxy->clear();
+$proxy->filter(\&inject_duplicate_extension_clienthello);
+$proxy->start();
+ok(TLSProxy::Message->fail(), "Duplicate ClientHello extension");
+
+$proxy->clear();
+$proxy->filter(\&inject_duplicate_extension_serverhello);
+$proxy->start();
+ok(TLSProxy::Message->fail(), "Duplicate ServerHello extension");
+
diff --git a/test/recipes/70-test_sslsessiontick.t b/test/recipes/70-test_sslsessiontick.t
index 8a361fd80387e4f119ccc6ae7c0edb00b3196a23..4e9c85f82fc47cc5b43a388a86f99b2cc71cc950 100755
--- a/test/recipes/70-test_sslsessiontick.t
+++ b/test/recipes/70-test_sslsessiontick.t
@@ -198,7 +198,7 @@ sub inject_empty_ticket_filter {
     foreach my $message (@{$proxy->message_list}) {
         push @new_message_list, $message;
         if ($message->mt == TLSProxy::Message::MT_SERVER_HELLO) {
-            $message->set_extension(TLSProxy::ClientHello::EXT_SESSION_TICKET, "");
+            $message->set_extension(TLSProxy::Message::EXT_SESSION_TICKET, "");
             $message->repack();
             # Tack NewSessionTicket onto the ServerHello record.
             # This only works if the ServerHello is exactly one record.
@@ -226,7 +226,7 @@ sub checkmessages($$$$$$)
 		#Get the extensions data
 		my %extensions = %{$message->extension_data};
 		if (defined
-                    $extensions{TLSProxy::ClientHello::EXT_SESSION_TICKET}) {
+                    $extensions{TLSProxy::Message::EXT_SESSION_TICKET}) {
 		    if ($message->mt == TLSProxy::Message::MT_CLIENT_HELLO) {
 			$chellotickext = 1;
 		    } else {
diff --git a/test/recipes/70-test_tlsextms.t b/test/recipes/70-test_tlsextms.t
index 82cb85676d18151a71f7c6529fc08b1d3ffdac48..022b3a8d6c5f643b8192ef801ac2aa2123761257 100644
--- a/test/recipes/70-test_tlsextms.t
+++ b/test/recipes/70-test_tlsextms.t
@@ -215,11 +215,11 @@ sub extms_filter
 
     foreach my $message (@{$proxy->message_list}) {
         if ($crmextms && $message->mt == TLSProxy::Message::MT_CLIENT_HELLO) {
-            $message->delete_extension(TLSProxy::ClientHello::EXT_EXTENDED_MASTER_SECRET);
+            $message->delete_extension(TLSProxy::Message::EXT_EXTENDED_MASTER_SECRET);
             $message->repack();
         }
         if ($srmextms && $message->mt == TLSProxy::Message::MT_SERVER_HELLO) {
-            $message->delete_extension(TLSProxy::ClientHello::EXT_EXTENDED_MASTER_SECRET);
+            $message->delete_extension(TLSProxy::Message::EXT_EXTENDED_MASTER_SECRET);
             $message->repack();
         }
     }
@@ -237,7 +237,7 @@ sub checkmessages($$$$$)
         #Get the extensions data
         my %extensions = %{$message->extension_data};
         if (defined
-            $extensions{TLSProxy::ClientHello::EXT_EXTENDED_MASTER_SECRET}) {
+            $extensions{TLSProxy::Message::EXT_EXTENDED_MASTER_SECRET}) {
             if ($message->mt == TLSProxy::Message::MT_CLIENT_HELLO) {
                 $cextms = 1;
             } else {
diff --git a/util/TLSProxy/ClientHello.pm b/util/TLSProxy/ClientHello.pm
index 72661129ae0f6dcc23d01f02d8320f7000e9b84f..383062842c8da49b1b50631144af097bf50b61a7 100644
--- a/util/TLSProxy/ClientHello.pm
+++ b/util/TLSProxy/ClientHello.pm
@@ -57,13 +57,6 @@ package TLSProxy::ClientHello;
 
 use parent 'TLSProxy::Message';
 
-use constant {
-    EXT_STATUS_REQUEST => 5,
-    EXT_ENCRYPT_THEN_MAC => 22,
-    EXT_EXTENDED_MASTER_SECRET => 23,
-    EXT_SESSION_TICKET => 35
-};
-
 sub new
 {
     my $class = shift;
@@ -90,7 +83,7 @@ sub new
     $self->{comp_meth_len} = 0;
     $self->{comp_meths} = [];
     $self->{extensions_len} = 0;
-    $self->{extensions_data} = "";
+    $self->{extension_data} = "";
 
     return $self;
 }
@@ -161,7 +154,7 @@ sub process_extensions
     #Clear any state from a previous run
     TLSProxy::Record->etm(0);
 
-    if (exists $extensions{&EXT_ENCRYPT_THEN_MAC}) {
+    if (exists $extensions{TLSProxy::Message::EXT_ENCRYPT_THEN_MAC}) {
         TLSProxy::Record->etm(1);
     }
 }
@@ -187,6 +180,11 @@ sub set_message_contents
         $extensions .= pack("n", $key);
         $extensions .= pack("n", length($extdata));
         $extensions .= $extdata;
+        if ($key == TLSProxy::Message::EXT_DUPLICATE_EXTENSION) {
+          $extensions .= pack("n", $key);
+          $extensions .= pack("n", length($extdata));
+          $extensions .= $extdata;
+        }
     }
 
     $data .= pack('n', length($extensions));
@@ -276,6 +274,11 @@ sub extension_data
     }
     return $self->{extension_data};
 }
+sub set_extension
+{
+    my ($self, $ext_type, $ext_data) = @_;
+    $self->{extension_data}{$ext_type} = $ext_data;
+}
 sub delete_extension
 {
     my ($self, $ext_type) = @_;
diff --git a/util/TLSProxy/Message.pm b/util/TLSProxy/Message.pm
index ddd0a6d3a840e4059be9fef94f0076da551e8b9a..bbb0ad70619a14038db824e001c51714c3ba293f 100644
--- a/util/TLSProxy/Message.pm
+++ b/util/TLSProxy/Message.pm
@@ -101,6 +101,16 @@ my %message_type = (
     MT_NEXT_PROTO, "NextProto"
 );
 
+use constant {
+    EXT_STATUS_REQUEST => 5,
+    EXT_ENCRYPT_THEN_MAC => 22,
+    EXT_EXTENDED_MASTER_SECRET => 23,
+    EXT_SESSION_TICKET => 35,
+    # This extension does not exist and isn't recognised by OpenSSL.
+    # We use it to test handling of duplicate extensions.
+    EXT_DUPLICATE_EXTENSION => 1234
+};
+
 my $payload = "";
 my $messlen = -1;
 my $mt;
diff --git a/util/TLSProxy/ServerHello.pm b/util/TLSProxy/ServerHello.pm
index 487538ab1286fcfe631b19f46bf663cb8b4159bf..7cf753508d1dda62473ddb8a2a0e9309fb8ff87a 100644
--- a/util/TLSProxy/ServerHello.pm
+++ b/util/TLSProxy/ServerHello.pm
@@ -80,7 +80,7 @@ sub new
     $self->{session} = "";
     $self->{ciphersuite} = 0;
     $self->{comp_meth} = 0;
-    $self->{extensions_data} = "";
+    $self->{extension_data} = "";
 
     return $self;
 }
@@ -161,6 +161,11 @@ sub set_message_contents
         $extensions .= pack("n", $key);
         $extensions .= pack("n", length($extdata));
         $extensions .= $extdata;
+        if ($key == TLSProxy::Message::EXT_DUPLICATE_EXTENSION) {
+          $extensions .= pack("n", $key);
+          $extensions .= pack("n", length($extdata));
+          $extensions .= $extdata;
+        }
     }
 
     $data .= pack('n', length($extensions));