add_role() run only once?

The user roles and capabilities are saved in the database so once you have you have used add_role() its saved and then next load WordPress will know that role just like the built in roles.

Now if you look at the function add_role() more specifically at line 141 you will see that it only saves the role and capabilities in the database if the var $use_db is set to true (which he is by default) so you can simply change it before you call your add_role() function and the role won’t be saved.

try:

//globalize $wp_roles
global $wp_roles;
//set use_db to flase
$wp_roles->use_db = false;
//then add your role
$wp_roles->add_role( $role, $display_name, $capabilities );

Update:

If its in a test/development environment then i see no downside, but if you are on a live environment then you save the time it take to create that on role every load.

As for best practice run once, if in a plugin you should use register_activation_hook and for any thing else i use a simple custom made conditional function:

function run_once($key){
    $test_case = get_option('run_once');
    if (isset($test_case[$key]) && $test_case[$key]){
        return false;
    }else{
        $test_case[$key] = true;
        update_option('run_once',$test_case);
        return true;
    }
}

**usage:**
if (run_once('add_user_role')){
    //do you stuff and it will only run once
}