That's not how dengue works. There are four variants of the dengue virus, and it gets worse the second time only if it's a different variant. Quoting Wikipedia (https://en.wikipedia.org/wiki/Dengue_virus):

"Developing a vaccine against the disease is challenging. With four different serotypes of the virus that can cause the disease, the vaccine must immunize against all four types to be effective. Vaccination against only one serotype could possibly lead to severe dengue hemorrhagic shock when infected with another serotype due to antibody-dependent enhancement. When infected with dengue virus, the immune system produces cross-reactive antibodies that provide immunity to that particular serotype. However, these antibodies are incapable of neutralizing other serotypes upon reinfection and actually increase viral replication."