A person can suffer from two types of cancer at the same time, which is clinically called double primary cancer or duplicated cancer. The recommended diagnostic criteria for double primary cancer are that both cancers are malignant, both cancers occur in different body parts that are not consecutive, and both cancers have their own unique morphological characteristics. In addition, each of the two cancers has its own unique metastatic pathway. The main reason for the occurrence of double primary cancer is the existence of immune deficiency and long-term exposure to cancer-causing environment, which makes the body prone to cancer due to the lack of immunity against multiple cancers. For double primary cancers, if the stage is early, radical surgery should be used as much as possible. For some patients who cannot have radical surgery, broad-spectrum chemotherapy drugs can be considered to take care of both cancers at the same time.