You appear to be loading in the file twice. Also you need to make sure that the function is being tied into a hook in order to make it load.
So the example direct from the WordPress codex is:
// Register style sheet.
add_action( 'wp_enqueue_scripts', 'register_plugin_styles' );
/**
* Register style sheet.
*/
function register_plugin_styles() {
wp_register_style( 'my-plugin', plugins_url( 'my-plugin/css/plugin.css' ) );
wp_enqueue_style( 'my-plugin' );
}
Taken from here: https://codex.wordpress.org/Function_Reference/wp_register_style
As you can see an action is declared and then we are loading our function into that action with a hook.
The function registers the script, including its file path, and then you only have to enqueue the style without declaring the file path again.
Secondly you can check the console for any errors pointing to incorrect file paths, or view the source-code and check the links in the header of the file to see if they are pointing to the correct place.
Another possible solution is that somehow, you are not declaring this function in the right place. All functions have to be placed within the functions.php and the function of wp_head() used in the header.php file.