diff --git a/include/coap3/coap_session_internal.h b/include/coap3/coap_session_internal.h
index bbca8e69..35bee756 100644
--- a/include/coap3/coap_session_internal.h
+++ b/include/coap3/coap_session_internal.h
@@ -149,6 +149,10 @@ struct coap_session_t {
   uint64_t tx_token;              /**< Next token number to use */
   coap_bin_const_t *last_token;   /** last token used to make a request */
   coap_bin_const_t *echo;         /**< Echo value to send with next request */
+  coap_mid_t last_ack_mid;        /**< The last ACK mid that has been
+                                       been processed */
+  coap_mid_t last_con_mid;        /**< The last CON mid that has been
+                                       been processed */
 };
 
 #if COAP_SERVER_SUPPORT
diff --git a/src/coap_session.c b/src/coap_session.c
index 6e80c381..e21f331b 100644
--- a/src/coap_session.c
+++ b/src/coap_session.c
@@ -209,6 +209,8 @@ coap_make_session(coap_proto_t proto, coap_session_type_t type,
   session->probing_rate = COAP_DEFAULT_PROBING_RATE;
   session->dtls_event = -1;
   session->last_ping_mid = COAP_INVALID_MID;
+  session->last_ack_mid = COAP_INVALID_MID;
+  session->last_con_mid = COAP_INVALID_MID;
 
   /* Randomly initialize */
   coap_prng((unsigned char *)&session->tx_mid, sizeof(session->tx_mid));
diff --git a/src/net.c b/src/net.c
index 9691bf23..82c56293 100644
--- a/src/net.c
+++ b/src/net.c
@@ -3033,6 +3033,23 @@ handle_response(coap_context_t *context, coap_session_t *session,
   if (rcvd->type != COAP_MESSAGE_ACK)
     coap_cancel_all_messages(context, session, rcvd->token, rcvd->token_length);
 
+  /* Check for message duplication */
+  if (COAP_PROTO_NOT_RELIABLE(session->proto)) {
+    if (rcvd->type == COAP_MESSAGE_CON) {
+      if (rcvd->mid == session->last_con_mid) {
+        /* Duplicate response */
+        return;
+      }
+      session->last_con_mid = rcvd->mid;
+    } else if (rcvd->type == COAP_MESSAGE_ACK) {
+      if (rcvd->mid == session->last_ack_mid) {
+        /* Duplicate response */
+        return;
+      }
+      session->last_ack_mid = rcvd->mid;
+    }
+  }
+
   if (session->block_mode & COAP_BLOCK_USE_LIBCOAP) {
     /* See if need to send next block to server */
     if (coap_handle_response_send_block(session, sent, rcvd)) {