Yes, dual citizenship (be a citizen of more than one country) is possible for a South African citizen. However, a South African citizen aged 18 or older must apply to the Department of Home Affairs for the retention of their citizenship in order to do so.
According to the South African Citizenship Act, you can only keep your South African citizenship if you apply for it before obtaining international citizenship. If a South African citizen may not seek this consent prior to willingly gaining international citizenship, they will immediately forfeit it.
In terms of the South African Citizenship Act, it is an offense for a South African Citizen holding dual citizenship to enter or depart South Africa using a passport of another country. This means that if you hold dual citizenship you must always enter and depart South Africa using your valid South African passport.
South African legislation allows a South African dual citizen to use their foreign passport freely outside of South Africa, however, in South Africa they are not allowed to use their foreign passport/citizenship to gain an advantage, avoid duty or responsibility that a South African Citizen would otherwise have/ have not been entitled or subjected to.