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Then wait with including header.php before you have determined the template.
Header.php basically initialize $xoopsTpl as a Smarty Template object, checks for cached pages and builds the blocks. If you provide a wrong template before including header.php, you risk the caching taking effect witht that template.
So as the first thing to do in a module page (after including mainfile.php naturally) is to figure out, which template is used - this may take function calls and other processing, but don't include header.php until this is done, and don't use the $xoopsTpl object until the header.php is included.
News v1.2 uses this in the main page to distinguish between By Topic and Classic view.