I ‘m not able to test if this actually works right now, but if you only need to duplicate the currently active page (i.e. not necessarily what you have selected), something like this might work:

var activeDocument = app.activeDocument
var activePage = app.activeWindow.activePage

activePage.duplicate()
activeDocument.pages[activeDocument.pages.length – 1].move(LocationOptions.AFTER, activePage)

Someone might find the following script useful.

It duplicates each page in place instead of appending the duplicates to the end of the document (e.g. 1-2-3 becomes 1-1-2-2-3-3).

—

// Define variables
var file = app.activeDocument;
var pages = file.pages;
var numberOfPages = pages.length;

// Duplicate each page
for (i = 0; i < numberOfPages; i++) {
file.pages[i].duplicate();
}

// Intertwine pages
for (var i = 0; i < (numberOfPages – 1); i++) {
var currentPage = (numberOfPages + i);
var insertPoint = (i + i);

file.pages[currentPage].move(LocationOptions.AFTER, file.pages[insertPoint]);
}

—