A Research Study to See How a Weekly Insulin, Insulin Icodec, Helps in Reducing the Blood Sugar Compared to Daily Insulin Glargine, Both in Combination With Insulin Aspart, in Adults With Type 1 Diabetes

Who May Qualify: - Diagnosed with type 1 diabetes mellitus greater than or equal to (≥) 1 year before screening. - Treated with multiple daily insulin injections (daily basal insulin analogue and bolus insulin analogue regimen) ≥ 6 months before screening. - HbA1c from 7.0-10.0 percentage (%) (53.0-85.8 millimoles per mole (mmol/mol)), both inclusive, at screening confirmed by central laboratory analysis. - Ability and willingness to adhere to the protocol including performance of self-measured plasma glucose (SMPG) profiles, based on the investigator's judgement. Who Should NOT Join This Trial: - Known or suspected hypersensitivity to study intervention(s) or related products. - Previous participation in this study. Participation is defined as signed willing to sign a consent form. - Female who is pregnant, breast-feeding or intends to become pregnant or is of childbearing potential and not using adequate contraceptive method. - Exposure to an investigational medicinal product within 90 days or 5 half-lives of the investigational medicinal product (if known), whichever is longer, before screening. - Any condition, except for conditions associated with type 1 diabetes mellitus, which in the investigator's opinion might jeopardise participant's safety or compliance with the protocol. - Anticipated initiation or anticipated change in concomitant medications (for more than 15 consecutive days) known to affect weight or glucose metabolism (e.g., treatment with thyroid hormones, or systemic corticosteroids). - Known hypoglycaemic unawareness as indicated by the Investigator according to Clarke's questionnaire question. - Recurrent severe hypoglycaemic episodes within the last year as judged by the investigator. Always talk to your doctor about whether this trial is right for you.