Custom Fields Adding Automatically

Hello ,

I have created a plugin which adds custom meta box in pages, it is working correctly but this also adds a custom field with the same name and value, can someone please help, is it the default wordpress property or i am doing something wrong, here is my code of plugin file,

add_action('add_meta_boxes','shm_add_meta');
if(!function_exists(shm_add_meta)){
	function shm_add_meta(){
		add_meta_box('subheading_id','Subheading For This Page','shm_create_meta','page',
		'side','high');
	}
}

if(!function_exists(shm_create_meta)){
  function shm_create_meta(){
	global $post;
	$values=get_post_custom($post->ID);
	if(isset($values['subheading_text'])){
		$text = esc_attr($values['subheading_text'][0]);
	}else{ $text = '';}

  wp_nonce_field('my_subheading_nonce','subheading_nonce');
  ?>
  <label for="subheading_text">Text For Subheading</label>
  <textarea rows="8" cols="30" name="subheading_text" id="subheading_text"><?php echo $text; ?></textarea>
  <?php
  }
}
add_action('save_post','shm_meta_save');
if(!function_exists(shm_meta-save)){
	function shm_meta_save($post_id){
		 // Bail if we're doing an auto save
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return; 

    // if our nonce isn't there, or we can't verify it, bail
    if( !isset( $_POST['subheading_nonce'] ) || !wp_verify_nonce( $_POST['subheading_nonce'], 'my_subheading_nonce' ) ) return; 

    // if our current user can't edit this post, bail
    if( !current_user_can( 'edit_post' ) ) return; 

   $allowed = array(
        'a' => array( // on allow a tags
            'href' => array() // and those anchors can only have href attribute
        )
    );
    if(isset($_POST['subheading_text']))
	update_post_meta($post_id,'subheading_text', wp_kses( $_POST['subheading_text'], $allowed ) );
	}
}

function show_shm_meta(){
global $post;
 $string = get_post_meta($post->ID,'subheading_text',true);
 if(!empty($string)) return $string;
 else return;

add_shortcode('display_subheadmeta','show_shm_meta');