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<?php

function get_custom_cat_template($single_template){
    global $post;
 
    if ( in_category( 'movies' )) {
    	$single_template = dirname( __FILE__ ) . '/single-movies.php';
    }
    return $single_template;
}
 
add_filter( "single_template", "get_custom_cat_template" ) ;

?>