If you have tested positive for coronavirus (COVID-19), you will probably have developed some immunity to the disease. But it cannot be guaranteed that will happen in all cases, nor exactly for how long that will last. There has been one report of a possible re-infection reported.
If you have previously tested positive but develop symptoms again, you must self-isolate for at least 7 days from onset of symptoms and be tested. If you live in a household, all other household members must stay at home for 14 days.
There is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection according to the WHO. However medical and scientific advisors to the government have suggested that based on the anti-body response to other viral infections including coronaviruses, it is unlikely that a person would be infected for a second time. However, the virus causing COVID-19 is so new that this needs to be further validated with evidence.