duplicatePage method
Implementation
Future<bool> duplicatePage(int index) async {
final List<UPdfRef> refs = await flattenPageTree();
if (index < 0 || index >= refs.length) return false;
final Object? page = await current(refs[index]);
if (page is! UPdfDict) return false;
final int number = allocate();
put(number, page.copy());
refs.insert(index + 1, UPdfRef(number, 0));
return setPageOrder(refs);
}