This study has three parts, called Sub-Studies. In Sub-Study 1, people receive a medicine called duvakitug. In Sub-Study 2, people are placed by chance into one of three groups and may get duvakitug, a placebo with no medicine, or both depending on the group. In Sub-Study 3, people are placed by chance into one of two groups to get different doses of duvakitug. The whole study lasts about thirty-five weeks and includes fifteen clinic visits. During these visits, people will have tests such as heart checks, blood tests, endoscopy, biopsies, and questionnaires. Depending on when a person joins, they will be placed in Sub-Study 1 or Sub-Study 2. In both of these, they will take the study drug every two weeks for twelve weeks. After finishing one of these parts, they may be asked to join Sub-Study 3 for another twelve weeks of treatment, or they may be asked to join a different study called STARSCAPE 2.