diff --git a/src/internal.c b/src/internal.c
index 44640fbad..f22fbd0c3 100644
--- a/src/internal.c
+++ b/src/internal.c
@@ -23355,6 +23355,9 @@ const char* wolfSSL_ERR_reason_error_string(unsigned long e)
     case DTLS_TOO_MANY_FRAGMENTS_E:
         return "Received too many fragmented messages from peer error";
 
+    case DUPLICATE_TLS_EXT_E:
+        return "Duplicate TLS extension in message.";
+
     default :
         return "unknown error number";
     }
diff --git a/src/tls.c b/src/tls.c
index 88d555e68..f37ec67b7 100644
--- a/src/tls.c
+++ b/src/tls.c
@@ -1246,6 +1246,8 @@ int TLS_hmac(WOLFSSL* ssl, byte* digest, const byte* in, word32 sz, int padSz,
  *   When adding a new extension type that don't extrapolate the number of
  *   available semaphores, check for a possible collision with with a
  *   'remapped' extension type.
+ *
+ * Update TLSX_Parse for duplicate detection if more added above 62.
  */
 static WC_INLINE word16 TLSX_ToSemaphore(word16 type)
 {
@@ -11918,10 +11920,14 @@ int TLSX_Parse(WOLFSSL* ssl, const byte* input, word16 length, byte msgType,
 #if defined(WOLFSSL_TLS13) && (defined(HAVE_SESSION_TICKET) || !defined(NO_PSK))
     int pskDone = 0;
 #endif
+    byte seenType[SEMAPHORE_SIZE];  /* Seen known extensions. */
 
     if (!ssl || !input || (isRequest && !suites))
         return BAD_FUNC_ARG;
 
+    /* No known extensions seen yet. */
+    XMEMSET(seenType, 0, sizeof(seenType));
+
     while (ret == 0 && offset < length) {
         word16 type;
         word16 size;
@@ -11942,6 +11948,22 @@ int TLSX_Parse(WOLFSSL* ssl, const byte* input, word16 length, byte msgType,
         ato16(input + offset, &size);
         offset += OPAQUE16_LEN;
 
+        /* Check we have a bit for extension type. */
+        if ((type <= 62) || (type == TLSX_RENEGOTIATION_INFO)
+        #ifdef WOLFSSL_QUIC
+            || (type == TLSX_KEY_QUIC_TP_PARAMS_DRAFT)
+        #endif
+            )
+        {
+            /* Detect duplicate recognized extensions. */
+            if (IS_OFF(seenType, TLSX_ToSemaphore(type))) {
+                TURN_ON(seenType, TLSX_ToSemaphore(type));
+            }
+            else {
+                return DUPLICATE_TLS_EXT_E;
+            }
+        }
+
         if (length - offset < size)
             return BUFFER_ERROR;
 
diff --git a/tests/api.c b/tests/api.c
index c3fa9d2b4..a9e20b923 100644
--- a/tests/api.c
+++ b/tests/api.c
@@ -9132,6 +9132,113 @@ static int test_wolfSSL_wolfSSL_UseSecureRenegotiation(void)
     return res;
 }
 
+#if !defined(NO_WOLFSSL_SERVER) && (!defined(NO_RSA) || defined(HAVE_ECC))
+/* Called when writing. */
+static int DummySend(WOLFSSL* ssl, char* buf, int sz, void* ctx)
+{
+    (void)ssl;
+    (void)buf;
+    (void)sz;
+    (void)ctx;
+
+    /* Force error return from wolfSSL_accept_TLSv13(). */
+    return WANT_WRITE;
+}
+/* Called when reading. */
+static int BufferInfoRecv(WOLFSSL* ssl, char* buf, int sz, void* ctx)
+{
+    WOLFSSL_BUFFER_INFO* msg = (WOLFSSL_BUFFER_INFO*)ctx;
+    int len = (int)msg->length;
+
+    (void)ssl;
+    (void)sz;
+
+    /* Pass back as much of message as will fit in buffer. */
+    if (len > sz)
+        len = sz;
+    XMEMCPY(buf, msg->buffer, len);
+    /* Move over returned data. */
+    msg->buffer += len;
+    msg->length -= len;
+
+    /* Amount actually copied. */
+    return len;
+}
+#endif
+
+/* Test the detection of duplicate known TLS extensions.
+ * Specifically in a ClientHello.
+ */
+static int test_tls_ext_duplicate(void)
+{
+    int res = TEST_SKIPPED;
+#if !defined(NO_WOLFSSL_SERVER) && (!defined(NO_RSA) || defined(HAVE_ECC))
+    static unsigned char clientHelloDupTlsExt[] = {
+        0x16, 0x03, 0x03, 0x00, 0x6a, 0x01, 0x00, 0x00,
+        0x66, 0x03, 0x03, 0xf4, 0x65, 0xbd, 0x22, 0xfe,
+        0x6e, 0xab, 0x66, 0xdd, 0xcf, 0xe9, 0x65, 0x55,
+        0xe8, 0xdf, 0xc3, 0x8e, 0x4b, 0x00, 0xbc, 0xf8,
+        0x23, 0x57, 0x1b, 0xa0, 0xc8, 0xa9, 0xe2, 0x8c,
+        0x91, 0x6e, 0xf9, 0x20, 0xf7, 0x5c, 0xc5, 0x5b,
+        0x75, 0x8c, 0x47, 0x0a, 0x0e, 0xc4, 0x1a, 0xda,
+        0xef, 0x75, 0xe5, 0x21, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x13, 0x01,
+        0x00, 0x9e, 0x01, 0x00,
+        /* Extensions - duplicate signature algorithms. */
+                                0x00, 0x19, 0x00, 0x0d,
+        0x00, 0x04, 0x00, 0x02, 0x04, 0x01, 0x00, 0x0d,
+        0x00, 0x04, 0x00, 0x02, 0x04, 0x01,
+        /* Supported Versions extension for TLS 1.3. */
+                                            0x00, 0x2b,
+        0x00, 0x05, 0x04, 0x03, 0x04, 0x03, 0x03
+        
+
+    };
+    WOLFSSL_BUFFER_INFO msg;
+    const char* testCertFile;
+    const char* testKeyFile;
+    WOLFSSL_CTX *ctx;
+    WOLFSSL     *ssl;
+
+#ifndef NO_RSA
+    testCertFile = svrCertFile;
+    testKeyFile = svrKeyFile;
+#elif defined(HAVE_ECC)
+    testCertFile = eccCertFile;
+    testKeyFile = eccKeyFile;
+#endif
+
+    ctx = wolfSSL_CTX_new(wolfSSLv23_server_method());
+    AssertNotNull(ctx);
+
+    AssertTrue(wolfSSL_CTX_use_certificate_file(ctx, testCertFile,
+        WOLFSSL_FILETYPE_PEM));
+    AssertTrue(wolfSSL_CTX_use_PrivateKey_file(ctx, testKeyFile,
+        WOLFSSL_FILETYPE_PEM));
+
+    /* Read from 'msg'. */
+    wolfSSL_SetIORecv(ctx, BufferInfoRecv);
+    /* No where to send to - dummy sender. */
+    wolfSSL_SetIOSend(ctx, DummySend);
+
+    ssl = wolfSSL_new(ctx);
+    AssertNotNull(ssl);
+
+    msg.buffer = clientHelloDupTlsExt;
+    msg.length = (unsigned int)sizeof(clientHelloDupTlsExt);
+    wolfSSL_SetIOReadCtx(ssl, &msg);
+
+    AssertIntNE(wolfSSL_accept(ssl), WOLFSSL_SUCCESS);
+    AssertIntEQ(wolfSSL_get_error(ssl, 0), DUPLICATE_TLS_EXT_E);
+
+    wolfSSL_free(ssl);
+    wolfSSL_CTX_free(ctx);
+
+    res = TEST_RES_CHECK(1);
+#endif
+    return res;
+}
 
 /*----------------------------------------------------------------------------*
  | X509 Tests
@@ -59358,6 +59465,7 @@ TEST_CASE testCases[] = {
 #endif
     TEST_DECL(test_wolfSSL_DisableExtendedMasterSecret),
     TEST_DECL(test_wolfSSL_wolfSSL_UseSecureRenegotiation),
+    TEST_DECL(test_tls_ext_duplicate),
 
     /* X509 tests */
     TEST_DECL(test_wolfSSL_X509_NAME_get_entry),
diff --git a/wolfssl/error-ssl.h b/wolfssl/error-ssl.h
index 8da041a24..94a5b97fa 100644
--- a/wolfssl/error-ssl.h
+++ b/wolfssl/error-ssl.h
@@ -181,6 +181,8 @@ enum wolfSSL_ErrorCodes {
     DTLS_CID_ERROR               = -454,   /* Wrong or missing CID */
     DTLS_TOO_MANY_FRAGMENTS_E    = -455,   /* Received too many fragments */
     QUIC_WRONG_ENC_LEVEL         = -456,   /* QUIC data received on wrong encryption level */
+
+    DUPLICATE_TLS_EXT_E          = -457,   /* Duplicate TLS extension in msg. */
     /* add strings to wolfSSL_ERR_reason_error_string in internal.c !!!!! */
 
     /* begin negotiation parameter errors */