Secondary listing means that a company has been listed on one exchange and then listed on another exchange, but the two exchanges do not conduct IPO at the same time. For example, a company has been listed in the United States, and now it is listed in Hong Kong for the second time. This is a secondary list.
Dual listing means that the same company is listed on two different exchanges at the same time. This means that the company's shares are traded on two exchanges, and the regulatory requirements of the two exchanges will be different. Companies listed in the two places need to comply with all relevant regulations of the two exchanges at the same time.