I solved this way:
add_action( 'woocommerce_add_cart_item', 'remove_duplicated_item' );
function remove_duplicated_item($var) {
global $woocommerce;
foreach( $woocommerce->cart->cart_contents as $key => $value ) {
if( $value['product_id'] == $var['product_id'] ) {
unset($woocommerce->cart->cart_contents[$key]);
return $var;
break;
}
}
return $var;
}