Check whether function exists or not

// check for function with the same name
if ( ! defined( 'ABSPATH' ) ) exit; // Exit if accessed directly
if ( ! function_exists( 'alex_vitorials_get_user_unique_name' ) ) {
	function alex_vitorials_get_user_unique_name(){
		if ( ! is_user_logged_in() ) {
			return; 
		}
			    else {
				$user = wp_get_current_user();
				$avatar = get_avatar( $user->ID, 40 );
				$user_nicename = '<span>';
				$user_nicename .= get_userdata( $user->ID )->user_nicename;
				$user_nicename .= '</span>';
				return $avatar . $user_nicename; 
	}
}
	add_shortcode( 'user-ID', 'alex_vitorials_get_user_unique_name' );
}