How often do people with cancer get cancer again in a different location, not related to the previous cancer?

Often. Unfortunately, the same factors that may have contributed to developing their first cancer may predispose cancer survivors to others. This may be related to both environmental factors (ie, assn between smoking and many different cancers) or hereditary factors (some cancer syndromes are linked to more than one cancer). In the latter scenario, genetic counseling may be very beneficial.
Can happen. As we slowly win the battle against cancer..People are surviving longer and living to older age. The older we are, the more chance we will develop cancer so it is not uncommon for someone, for example, who had breast cancer 30 years ago to develop skin cancer or another unrelated cancer.
Why do you ask? Tobacco users and people with cancer family syndromes are more prone than others to get second primaries because of the increased overall risk. By contrast, people with normal baseline risk have little extra risk at a different location. Nowadays the lifetime risk of getting cancer is about 2 in 5, and of dying of it about 1 in 5.