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header.php is the valid file showing the head of your page.
header.phpxxx is simply a replacement file, so that in case you modify the header.php you have the option to save the original(header.php) file and replace it for the replacement file (header.phpxxx).
dunno if i have explained. 
header.php is the valid file showing the head of your page.
header.phpxxx is simply a replacement file, so that in case you modify the header.php you have the option to save the original(header.php) file and replace it for the replacement file (header.phpxxx).
dunno if i have explained.