If the specific widget does not support having more-than-one, then you can’t have more-than-one.
Simple as that, really.
If it’s an old widget plugin, then the format for the multiple widget interface changed, and the plugin will need to be updated. A widget can either support 1 copy of itself or unlimited copies of itself now, there’s no need to specify a specific number any more.
You cannot duplicate the pages widget. There’s no duplicating widgets any more, what you do now is keep adding as many as you need. So the Text widget for example, you add as many widgets as you want, there’s no limit. But you don’t need to decide in advance how many you want (that was the old system).
If you want a second Pages widget, I wrote a really simple plugin to do that. However, it will only work properly if the widgets are on different sidebars. More specifically, you can only save the options for the widget if the two widgets are in different sidebars. If they are in the same sidebar, they’ll both get the same options when you click save.