We have known for quite a long time that it's possible to be infected with HIV more than once. Dual HIV infections were first reported in 2002.
However recombinant HIVs were detected long before.
Recombinant HIVs are viruses that are derived from at least two different HIVs; parts of each join together to produce a new virus. Since this can only happen in a cell infected with both viruses, the existence of recombinant HIVs means that infection with more than one HIV must have occurred.
We don't know how frequently this occurs. Some people will have been infected with more than one virus at the same time. But others already infected may be subsequently superinfected, that is infected with a different HIV.
The current issue of Clinical Infectious Diseases has an article on dual HIV infection.
It demonstrates that dual infection in one particular cohort was associated with a more rapid decline in CD4 numbers.
In the authors' own words:
Conclusions. Dual HIV-1 infection is the main factor associated with CD4+ T-cell decline in men who have untreated primary infection with HIV-1 subtype B.
This conclusion is based on the study of only 10 men dually infected, drawn from an already selected group of 37 untreated men who were followed after a primary HIV infection.
Dual infection has also been reported among long term non-progressors, so it's impossible at this time to generalize about its effects.
The article demonstrates that dual infection may not be that uncommon. It was detected in 10 of a group of 37, albeit selected, men.
Superinfection in treated individuals may result in a failure of particular drug regimen should the superinfecting virus be resistant to it.
Even if we still have much to learn about dual infections, the implications regarding continued condom use among seropositive individuals are clear enough, a point made by the authors.