update_user_meta duplicates entry

You are overwriting the existing values instead you should check if existing values exist and concatenate the new values to existing values if it is exist as following

if (is_user_logged_in()) {
 $user = wp_get_current_user();
 $aka2 = get_user_meta($user->ID, 'last_visited_blogs',true);
  if($aka2)
 array_push ($aka2, 'blog4');
 else
 $aka2 = array('blog4');
  update_user_meta($user->ID, 'last_visited_blogs', $aka2);
}