How to upload multiple images on frontend to ACF gallery using update_field

I’m trying to save images submitted on a frontend form in a way that they are accessible via the ACF backend.

In how to upload an image on ACF with update_field on wordpress, someone provided a solution for a single image upload (and consequently a single ACF image field). I thought it should be pretty easy extending this to a multi image upload (and a ACF gallery field), but I get an error I can’t track down:
Cannot redeclare wp_crop_image() in /../travel/wp-admin/includes/image.php on line 25

in functions.php

function RemapFilesArray($name, $type, $tmp_name, $error, $size)
{
    return array(
        'name' => $name,
        'type' => $type,
        'tmp_name' => $tmp_name,
        'error' => $error,
        'size' => $size,
    );
}

function my_update_attachment($f,$pid,$t='',$c='') {
  wp_update_attachment_metadata( $pid, $f );
  if( !empty( $f['name'] )) { //New upload
    require_once( ABSPATH . 'wp-admin/includes/file.php' );
    include( ABSPATH . 'wp-admin/includes/image.php' );

    $override['test_form'] = false;
    $file = wp_handle_upload( $f, $override );

    if ( isset( $file['error'] )) {
      return new WP_Error( 'upload_error', $file['error'] );
    }

    $file_type = wp_check_filetype($f['name'], array(
      'jpg|jpeg' => 'image/jpeg',
      'gif' => 'image/gif',
      'png' => 'image/png',
    ));
    if ($file_type['type']) {
      $name_parts = pathinfo( $file['file'] );
      $name = $f['name'];
      $type = $file['type'];
      $title = $t ? $t : $name;
      $content = $c;

      $attachment = array(
        'post_title' => $title,
        'post_type' => 'attachment',
        'post_content' => $content,
        'post_parent' => $pid,
        'post_mime_type' => $type,
        'guid' => $file['url'],
      );


      foreach( get_intermediate_image_sizes() as $s ) {
        $sizes[$s] = array( 'width' => '', 'height' => '', 'crop' => true );
        $sizes[$s]['width'] = get_option( "{$s}_size_w" ); // For default sizes set in options
        $sizes[$s]['height'] = get_option( "{$s}_size_h" ); // For default sizes set in options
        $sizes[$s]['crop'] = get_option( "{$s}_crop" ); // For default sizes set in options
      }

      $sizes = apply_filters( 'intermediate_image_sizes_advanced', $sizes );

      foreach( $sizes as $size => $size_data ) {
        $resized = image_make_intermediate_size( $file['file'], $size_data['width'], $size_data['height'], $size_data['crop'] );
        if ( $resized )
          $metadata['sizes'][$size] = $resized;
      }

      $attach_id = wp_insert_attachment( $attachment, $file['file'] /*, $pid - for post_thumbnails*/);

      if ( !is_wp_error( $attach_id )) {
        $attach_meta = wp_generate_attachment_metadata( $attach_id, $file['file'] );
        wp_update_attachment_metadata( $attach_id, $attach_meta );
      }

   return array(
  'pid' =>$pid,
  'url' =>$file['url'],
  'file'=>$file,
  'attach_id'=>$attach_id
   );
    }
  }
}

on my form.php:

<input type="file" name="upload_attachments[]">
<input type="file" name="upload_attachments[]">

in process-form.php

if ($_FILES['upload_attachment']){

 $files = array_map('RemapFilesArray',
 $_FILES['upload_attachment']['name'],
 $_FILES['upload_attachment']['type'],
 $_FILES['upload_attachment']['tmp_name'],
 $_FILES['upload_attachment']['error'],
 $_FILES['upload_attachment']['size']
);

$gallery=array();

foreach ($files as $file){ //loop through each file
    $att = my_update_attachment($file,$site_id);
    array_push($gallery,$att['attach_id']);
}
update_field('field_1231242',$gallery,$site_id);

}

I appreciate your help and ideas!

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