Additional Study Information: The purpose of this study is to test the study drug, BI 907828 for safety, effectiveness, and side effects when compared to receiving the standard treatment, doxorubicin, in subjects with dedifferentiated liposarcoma (DDLPS). BI 907828 has proven to minimize a critical protein interaction between MDM2 and p53, two proteins in your body, which when their interaction is diminished, can stop the growth of your cancer. Patients with DDLPS are normally prescribed Doxorubicin as their standard treatment once they are diagnosed. The study also aims to find the recommended dose of the study drug, BI 907828 that can be used in the second part of this study (Part 2) and in future clinical research studies. We will ask you to take the study drug orally or take the FDA-approved drug, doxorubicin, intravenously (in the vein). The time frame for the study depends on how long you decide to stay on BI 907828, the study drug.