Whether or not you can get the virus twice is a live question at the moment. There is a little bit of evidence that you can be reinfected with it which is a very scary prospect if it’s true. Because it could mean that if there is a vaccine, the vaccine might not last that long. But there is a little bit of evidence just on the other side of this reinfection story that it may actually not be reinfection and it’s a problem with the testing.
The tests are not perfect and they can miss the infection. So let’s say that you’ve got the infection, you’ve known it and then you test negative and then a week later you test positive. It’s quite likely that’s a problem with the test rather than reinfection, that it was a testing error that showed you as negative because the test couldn’t detect tiny quantities of the virus whereas another test did. So at the moment the betting is on no reinfection but there are worrying signs around that it’s possible.