Only the server can flip users.

network/netplay/netplay_frontend.c

@@ -969,6 +969,8 @@ bool netplay_driver_ctl(enum rarch_netplay_ctl_state state, void *data)
          goto done;
       case RARCH_NETPLAY_CTL_FLIP_PLAYERS:
          {
+            if (!netplay_data->is_server)
+               return false;
             bool *state = (bool*)data;
             if (*state)
                netplay_flip_users(netplay_data);