`command_process_create_cell` tries to check several conditions and can to call `channel_send_destroy` for already exist circuit. `channel_send_destroy` is more complex mechanism than sending of some cell as it happened before. It shouldn't to call `channel_send_destroy` if `circ_id` already exist for `chan`. It should to check `circ_id` and drop if already exist and only then to check all another stuff.