A mark can be used only once

Unfortunately this fix breaks two of my slide decks:
- `samples/pyconlt/talk.mgp`
- `samples/pyconlt/python-lt.mgp`

They're quite broken when I try to view them with `mgp`, so I should try to fix them too.  Which is harder than it sounds, especially given that my handling of %again isn't quite correct, even after this fix.