In practice you'd be fine. Most of the JNB immigration have no idea what they're doing, so just ask them to stamp on a page that you've already got stamps on.

One colleague has taken to putting post-it notes on all free pages saying "leave this page blank", which works for them, although a bit punchy for my tastes.

There's always a risk in South Africa that something random happens and you end up in trouble with the authorities (the police just outside the airport like telling people that foreign drivers licences are not allowed, which is crap).