SOLVED: how to avoid duplicate drupal error messages?

Intialize the Drupal session array.
$_SESSION['messages']['error'] = array();

After initializing, you need to set the drupal error message. By using this way u can avoid duplicate of drupal error messages.

Example: 
drupal_set_message(t("No Records found!"), 'error');   

[SOLVED] An illegal choice has been detected. Please contact the site administrator.

Try adding ‘#validated’ => ‘TRUE’ to form elements definitions:

form['contentlist'] = array(
‘#type’ => ‘select’,
‘#validated’ => TRUE
…
)

[SOLVED] HTML Parsing Error: Unable to modify the parent container element before the child element is closed (KB927917) Line: 0 Char: 0 Code: 0

Message: HTML Parsing Error: Unable to modify the parent container element before the child element is closed (KB927917) Line: 0 Char: 0 Code: 0

If you face this issue. Just do the below steps.

1. Check whether you have any Popup page in you website.
    Like Thickbox, Lightbox, etc..

2. Don't open the popups directly in the pages.

3. Open the Popups inside the Document Ready state.
   Example:

$(document).ready(function(){
  open_thickbox('logout/popup', 450, 600, true, true);
});

4. This solution for IE related browsers only. Because Firefox and other browsers will not trigger this issue.

5. This solution for IE7, IE8 etc..

6. Enjoy.. ! If you still face issue .. Please post comments here.. !!