additional image sizes

Hi,
for four days I have been looking for a way to add image sizes to wordpress.
wordpress comes with thumb, medium, large and full and I need four more to be able to select from, i.e. I need to be able to select from seven options when inserting an image into a page. To make it clear: I am not looking for support for a featured image and I do not want to use a plugin.

I found the following code on the internet – the folloeing is meant for the
functions.php of the child theme and I inserted the added images:

add_action( 'after_setup_theme', 'setup' );

function setup() {
	// ...
	
	add_theme_support( 'post-thumbnails' ); // This feature enables post-thumbnail support for a theme
	// To enable only for posts:
	//add_theme_support( 'post-thumbnails', array( 'post' ) );
	// To enable only for posts and custom post types:
	//add_theme_support( 'post-thumbnails', array( 'post', 'movie' ) );
	
	// Register a new image size.
	// This means that WordPress will create a copy of the post image with the specified dimensions
	// when you upload a new image. Register as many as needed.
	// Adding custom image sizes (name, width, height, crop)
	add_image_size( 'smallest', 479, 157, true );		
	add_image_size( 'small', 768, 252, true );	
	add_image_size( 'small_medium', 981, 322, true );
	add_image_size( 'normal_medium', 1536, 504, true );	
	add_image_size( 'better', 1962, 644, true );
	add_image_size( 'normal_large', 2304, 768, true );
	
	// ...
}

add_filter( 'image_size_names_choose', 'custom_image_sizes_choose' );
function custom_image_sizes_choose( $sizes ) {
	$custom_sizes = array(
	'smallest' => 'Smallest',
	'small' => 'Small',	
	'small_medium' => 'Small Medium',
	'normal_medium' => 'Medium',	
	'better' => 'Better',
	'normal_large' => 'Large'
	);
	return array_merge( $sizes, $custom_sizes );
}

PHP is completely new to me, but the the add_image part coincides with info from the wordpress codex and the add_filter is meant to let you select the image sizes in the wordpress gui.

But it is not working properly:
the upload directory contains the right image sizes but if I try to add an image to a page the amount of image sizes and their names are correct but some image sizes appear twice or more.

As code for the loop i copied the index.php to my child theme folder and added
following line

<?php
      if ( has_post_thumbnail()):
	  the_post_thumbnail( 'smallest', array( 'class' => 'smallest' ) );
          the_post_thumbnail( 'small', array( 'class' => 'small' ) );
	  the_post_thumbnail( 'small_medium', array( 'class' => 'small_medium' ) );
	  the_post_thumbnail( 'normal_medium', array( 'class' => 'normal_medium' ) );
	  the_post_thumbnail( 'better', array( 'class' => 'better' ) );
	  the_post_thumbnail( 'normal_large', array( 'class' => 'normal_large' ) );
                    endif;
                    ?>

I looked for following code snippet in my parent theme (Divi):

add_action( 'after_setup_theme'

but as I could not find it i added it to my child theme.

I would be very glad for some help as the more I read in the internet the more confused I get.

thanks in advance