Just means adding more to something that already exists, so that is full/full strength. So in this instance someone could have the coronavirus and be fairly asymptomatic or with mild symptoms having caught it from one source, then whilst the body is dealing with the virus it is caught again from somebody else where it might be slightly different or even the same, starting the cycle again, this could even happen several times. There was a woman in Japan who caught it a second time having well cleared the incubation period.
Basically we don't know enough about it at this stage to say things with enough certainty.