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<?php
/**
 * Change the add to cart text on single product pages
 */
function woo_custom_cart_button_text() {

  global $woocommerce;
	
	foreach($woocommerce->cart->get_cart() as $cart_item_key => $values ) {
		$_product = $values['data'];
	
		if( get_the_ID() == $_product->id ) {
			return __('Already in cart - Add Again?', 'woocommerce');
		}
	}
	
	return __('Add to cart', 'woocommerce');
}
add_filter('single_add_to_cart_text', 'woo_custom_cart_button_text');
add_filter( 'add_to_cart_text', 'woo_custom_cart_button_text' );
?>