Africa terrestrial-border crossings are still quite formal affairs. They often can take up to 30 minutes or more. It usually goes something like this:
1. Park car and queue for immigration exit stamp.
2. Go through exit customs procedure and sometimes receive exit permit.
3. Occasionally have vehicle and its contents inspected by customs agents.
4. Drive across border (hand in exit permit if required).
4. Park park across border and queue for entry visa and/or stamp.
5. Customs processing by the entering nation.
6. Sometimes vehicle inspection including showing proof of insurance.
7. In the meantime, others in your party may exchange currency on the black market, buy lunch etc. etc.
So you usually get two stamps, an exit and entry stamp, when crossing many African land borders. On more remote crossings, they tend to be extremely thorough. It once took us 80 minutes to cross into Zimbabwe from Mozambique.