ST: Well, it is difficult to prove reinfection. This is

because in general the fitter virus tends to predominate and 'drown out' the signal from the less fit one. Also, because drug-resistant viruses are usually the less fit ones, you may not be able to detect them while someone's not taking anti-HIV therapy. It's only when they start treatment and the resistant type 'pops up' that it becomes clear it was there all along, perhaps inactive inside cells.
I do think reinfection maybe happens more commonly than we believe. We assume it must happen, because of so-called recombinant HIV viruses. These are ones whose genetic material is a mix of different strains. This can only happen where there are two different subtypes of HIV existing together in the same cell.
But it's by no means clear how often reinfection has clinical consequences.
One other thing we've discovered is that AZT-resistant HIV can 'masquerade' as wild-type, drug-sensitive virus. If you're infected with AZT-resistant virus it can revert in your body to an apparently drug-sensitive type. But it's not really: it's only one chemical step away from the resistant type, whereas true wild-type virus is two steps away. So it takes very little in the shape of imperfect drug adherence for it to go back to being resistant if you start AZT.
GC: The two partners could say: 'OK, but we both have undetectable viral loads, so reinfection doesn't apply'.

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ST: Undetectable in the blood doesn't mean undetectable in semen or genital fluids. For a start, even when there is no free virus in the seminal