It is interesting, that someone can do this by hand or with scripting. I like the idea of somewhat “half automatism” ;-)
So, assume you have a single page on the right, then facing pages, e.g. 500. I would do the following:
1. Create a new list for numbers (solves the problem of threading)
2. Create on page 2 and 3 one textframe, insert a return and apply your list numbering (the contents show now 1 and 2)
3. Group those 2 frames and select it
4. Run the following lines:
var sel = app.selection[0];
var allSpreads = app.activeDocument.spreads;
var n = allSpreads.length;
while (n --) {
sel.duplicate(allSpreads[n]);
}
5. Delete the selected group on page 2+3
6. Ungroup the group on page 1 and delete the left frame
Why do I like this idea?
Cause otherwise a script must create a list, create frames, check if left or right handed, insert content … this would take much longer as the above 6 steps.