gnrc_sixlowpan_frag: drop received duplicates

Otherwise duplicate fragments are added to the number of already
received datagram bytes in L153 leading to possible race-conditions.

sys/net/gnrc/network_layer/sixlowpan/frag/rbuf.c

@@ -139,7 +139,12 @@ static int _rbuf_add(gnrc_netif_hdr_t *netif_hdr, gnrc_pktsnip_t *pkt,
              * https://tools.ietf.org/html/rfc4944#section-5.3 */
             return RBUF_ADD_REPEAT;
         }
-
+        /* End was already checked in overlap check */
+        if (ptr->start == offset) {
+            DEBUG("6lo rbuf: fragment already in reassembly buffer");
+            gnrc_pktbuf_release(pkt);
+            return RBUF_ADD_SUCCESS;
+        }
         ptr = ptr->next;
     }