Unless I'm missing something, the only way to duplicate pages is to import the PDF file multiple times and delete the pages you only want once. The attached patch adds that capability. When multiple pages are selected, it inserts the duplicates after the final page of the selection. This makes sense for contiguous selections, perhaps less so for non-contiguous selections. This patch takes advantage of the observation that the last page of the selection is the first returned in iconview.get_selected_items(). But I don't know if this behavior is guaranteed or just an accident.