Fix nsapi_dns_add_server to add a server just once

features/netsocket/nsapi_dns.cpp

@@ -131,6 +131,13 @@ static bool dns_timer_running = false;
 // DNS server configuration
 extern "C" nsapi_error_t nsapi_dns_add_server(nsapi_addr_t addr, const char *interface_name)
 {
+    // check if addr was already added
+    for (int i = 0; i < DNS_SERVERS_SIZE; i++) {
+        if (memcmp(&addr, &dns_servers[i], sizeof(nsapi_addr_t)) == 0) {
+            return NSAPI_ERROR_OK;
+        }
+    }
+
     memmove(&dns_servers[1], &dns_servers[0],
             (DNS_SERVERS_SIZE - 1)*sizeof(nsapi_addr_t));