wp_insert_post inserts 3 posts at once?

Thanks to @TomJNowell’s hint I managed to get it working with a simple check if the post already exists (using the post_exists function). This code is included in the functions.php file.

Additionally, the loop only runs if called from the backend using the is_admin function.

Here’s the working example:

function ekesto_insert_post(){

    // config
    $post_type="event";
    
    // post_exists() is by default only available in the admin, so let's get it in case the page is loaded on the frontend
    require_once(ABSPATH . 'wp-admin/includes/post.php');

    $import_array = array();

    $new_post = array(
        'post_title' => 'Test Event',
        'post_status'  => 'publish',
        'post_type'  => $post_type
    );
    $new_post_custom_fields = array(
        'category' => 29,
        'date_start' => '20231101',
        'venue_name' => 'Schloss Glarisegg',
        'location' => 'Steckborn (ZH)',
        'organizer_name' => 'Pathways Courses',
        'organizer_url' => 'https://pathways-courses.org/'
    );
    $import_array[] = array($new_post, $new_post_custom_fields);

    // only run code in the backend
    if( is_admin()) {
        foreach ($import_array as $array) {
            // check if post title exists
            $fount_post = post_exists( $array[0]['post_title'],'','',$post_type,'publish');
            // 0 is returned if post doesn't exist yet
            if ($fount_post === 0) {
                // Insert post into the database
                $post_id = wp_insert_post($array[0], true); // Use $wp_error set to true for error handling
                // Fill Advanced Custom Fields (ACF) fields
                foreach ($array[1] as $key => $value) {
                    update_field($key, $value, $post_id);
                }
                // Check if there was an error during post insertion
                if (is_wp_error($post_id)) {
                    // Error occurred while inserting the post
                    echo "Error: " . $post_id->get_error_message();
                } else {
                    // The post was successfully inserted, and $post_id contains the post ID
                    echo "Post inserted successfully. New Post ID: " . $post_id;
                }
            }
        }
    }    
}

add_action('after_setup_theme', 'ekesto_insert_post');

Note: The code also adds Advanced Custom Fields (ACF) data using the ACF function update_field().