Not a doctor and don't spend so much time on it. I think "the flu" is really just a category of infectious disease. You can't get "the flu" again, if it's the same virus. But you can get "the flu" again if it's a different one. Since viruses are constantly evolving, there are new ones every year.
So if you get flu-23A in the year 2025, you can't get flu-23A for a second time. But you might be able to get flu-23B and you can definitely get flu-24A in 2026. Or something a bit like that.
I don't know how quickly COVID-19 will evolve. Maybe there are 18 COVIDs before this one. But this COVID is as infectious as hell and overwhelms health systems.