Does anybody know how to exactly duplicate a page inside the same file? The standard copy-and-paste sequence will not do, since the stuff you paste into the destination page does not appear in the very same place as in the source page. Having some workaround would be useful to produce PDF presentations with overlays (Jarnal can do this, but the quality of its PDF renderings sucks compared to xournal…).
I'm afraid it's not currently possible to do an exact duplication of a page. (Eventually it will be possible to copy-paste entire pages, I hope, but not right now).
If you don't mind a hack, you can always gunzip the .xoj file, open it in a text editor, and duplicate the entire
"<page> .. </page>" section corresponding to the page you want to duplicate, then save again. Not very user-friendly, but should work.
Make a backup copy of the file first, since the file loading code is very finicky and will refuse to open the file if anything's wrong.