Double certificate number refers to two kinds of signed SSL certificates: ECC and RSA. RSA certificate is very common because it is very compatible and practical. ECC certificates are faster, but older browsers don't support them. Therefore, in order to improve the browsing speed of users, some websites will register two certificates with different encryption types and deploy them together to increase the user experience.

Two different brands of SSL certificates are deployed on the same server at the same time, and the main and backup certificates are online at the same time, so the double guarantee is more reliable. When an SSL certificate of one brand has an operation or security accident, another certificate can ensure the continuity of the website authentication service.