This can make you sicker faster if the new strain is more aggressive than the strain you already have. If the new strain is resistant to HIV drugs, it may also stop your drugs from working or limit the therapy you take in future.

It is hard to say how common re-infection is, as it has not been seen in large numbers. If you have HIV you will need to find out as much as you can and make your own decision.

You are more likely to re-infect others if you have a detectable viral load, and it is more likely the higher your viral load is.
HIV specialists can give advice on this.

Compared to the risk of re-infection, picking up a sexually transmitted infection (STI) from having unprotected sex is much more likely to happen. You might think that once you have HIV the worst has happened, but unprotected sex can give you infections such as herpes, syphilis, gonorrhoea and Chlamydia and hepatitis C.