Why is there 2 entries in the extension table when installing the OpenCart module?

Question

opencart

When trying to install a written module in OpenCart 1.5.5.1 with the setup function:
public function install(){
$this->load->model('setting/extensions');
$this->model_setting_extensions->install('total', 'my_module');
}
2 rows 'total', 'my_module' are inserted into the extension table
?
Exactly the same effect, if you do not write an installation function at all.
----------
UPDATE 10.24 10/30/2016
When installed on 1.5.6 (specially downloaded and tested), it is installed correctly.
However, I'm running the site on version 1.5.5.1 and there are plugins installed and an update is not in the plans. What can be done specifically for this version so that the installation works correctly?