Form submission with a ?key=val query string already in the action attribute ignores that query string

I know this is an old question, but the solution is actually pretty simple (and neat!).

All you have to do is sending the querystring with hidden input fields in the format name="key" and value="value".

?brandcode=JM&t=cat_items would "translate" into:

<input type="hidden" name="brandcode" value="JM" />
<input type="hidden" name="t" value="cat_items" />

Completely remove the querystring from your action.