I doubt, based on what many experts are saying, people are infected a second time with covid19 unless they have an impaired immune system.
I would guess the persons impacted aren't clearing out the infected cells in their body. Viruses can linger in cells for a long time. Since this virus seems to target and infect epithelial cells, people probably end up with pockets of infected cells throughout the body that can take a looooong time to clear out.
Here is an article on this (produced about viruses in general, not our current coronavirus).
https://www.sciencedaily.com/releases/2017/10/171006085921.htm
This means that individuals like this likely aren't infectious for others, but are still dealing with the effects of their first infection.
As a note, I'm not an expert. I just read a lot and this is a fascinating topic.