Use same slide number on several slides in keynote?

I don't think there is an option for this. So you will have to manually number all slides in order to keep the slide/page number the same for several of them.

Maybe the answer is to resize the notes display in the Presenter View. If you can get your notes to fit comfortably in Presenter View, then there is little need for duplicating the slides.

Try the following:

1. In Keynote, select "Play -> Customize Presenter Display". You'll be presented with a flexible layout and and an Inspector
2. In the inspector, assure that "Notes" is checked.
3. Try resizing the "Notes" display so that it is large enough to view all (or most) of your notes. (You can have only the notes displayed if you like.)

Everything on the Presenter display is moveable, so try editing all the elements to suit your needs.