[Done v1.6.0] Advanced Search Result alpha_filter_id included twice in GET parameters

1. Search for DVD.
   Address is:

.../zc151/index.php?main_page=advanced_search_result&search_in_description=1&keyword=dvd

2. Select the alpha sort for A.
   Address is:

.../zc151/index.php?main_page=advanced_search_result&search_in_description=1&keyword=dvd&inc_subcat=0&sort=21a&alpha_filter_id=65

3. Select the alpha sort for B.

.../zc151/index.php?main_page=advanced_search_result&search_in_description=1&keyword=dvd&inc_subcat=0&sort=21a&alpha_filter_id=65&alpha_filter_id=66

4. Select the alpha sort for C.

.../zc151/index.php?main_page=advanced_search_result&search_in_description=1&keyword=dvd&inc_subcat=0&sort=21a&alpha_filter_id=66&alpha_filter_id=67

I think in header.php,
change

 if ( ($key != 'currency') && ($key != zen_session_name()) && ($key != 'x') && ($key != 'y') ) {

to

 if ( ($key != 'currency') && ($key != zen_session_name()) && ($key != 'x') && ($key != 'y') && ($key != 'alpha_filter_id') ) {

Actually, the code change you suggested is not in header_php at all, but rather in the template file. So, for v1.5.1 and older, your fix could be applied to tpl_advanced_search_result_default.php.

But, the better fix is actually broader and is incorporated in the following commit to v1.6.0:
https://github.com/zencart/zencart/commit/0b217e697255fd8e70d1a7047bf1824b37892ef6

and these subsequent tweaks:
https://github.com/zencart/zencart/commit/3791bbede7b38650ef278e07fbac53d483426a2e
https://github.com/zencart/zencart/commit/71aa00a98818a4e965ba2a90ec4cb9862fdf95c4
https://github.com/zencart/zencart/commit/01a3db6035baa9357c9697e559db9b1957134d44