Scientists at the University of Hong Kong (UHK) said genetic sequencing has proved the IT worker contracted Covid-19 strains in April and August that were "clearly different".
Initially the health officials were unsure if the man was a "persistent carrier" of the virus from his first infection in April.
They described him as an "apparently young and healthy patient" whose second infection was detected at an airport screening following a trip to Spain in August.
“Many believe that recovered Covid-19 patients have immunity against re-infection because most developed a serum neutralising antibody response," said the authors of the report.
"However, there is evidence that some patients have waning antibody levels after a few months.
“Our findings suggest that the SARS-CoV-2 may persist in the global human population as is the case for other common cold-associated human coronaviruses, even if patients have acquired immunity via natural infection."
The researchers added that the study was accepted by the Clinical Infectious Diseases journal on Monday.
The man's second infection was asymptomatic, meaning he did not display any symptoms of coronavirus.
"Since the immunity can be short-lasting after natural infection, vaccination should also be considered for those with one episode of infection," said the researchers.
Speaking about the implications of the discovery, they explained: "It is unlikely that herd immunity can eliminate SARS-CoV-2, although it is possible that subsequent infections may be milder than the first infection as for this patient.
"Second, vaccines may not be able to provide lifelong protection against Covid-19. Furthermore vaccine studies should also include patients who have recovered from Covid-19."
Previous studies have suggested immunity to Covid-19 in recovered patients may only last a few months.
According to research by King's College London, which has not yet been peer-reviewed, immunity antibodies decrease significantly in the three months following infection leaving patients susceptible to reinfection year after year – similar to the common cold.
Experts pointed out that although the study is yet to be peer-reviewed, it should be a warning to those assuming they are immune if they have been infected with Covid-19 in the past.
Mala Maini, professor of viral immunology and consultant physician at University College London, said: “This study does reinforce the message that we can’t assume someone who has had Covid-19 can’t get it again just because they initially became antibody-positive.
“It also means a negative antibody test now can’t exclude you having had Covid-19 a few months ago.
“And it suggests vaccines will need to be better at inducing high levels of longer-lasting antibodies than the natural infection, or that doses may need to be repeated to maintain immunity.”