A Trial Comparing Insulin Degludec/Liraglutide and Insulin Degludec in Combination With Metformin in Chinese Subjects With Type 2 Diabetes Mellitus Inadequately Controlled With Basal Insulin Therapy and Metformin With or Without One Other Oral Antidiabetic Drug (OAD)

Inclusion Criteria: Informed consent obtained before any trial-related activities. Trial-related activities are any procedures that are carried out as part of the trial, including procedures to determine suitability for the trial - Male or female, age at least 18 years at the time of signing inform consent - Type 2 diabetes mellitus (clinically diagnosed) - HbA1c (glycosylated haemoglobin) above or equal to 7.5% by central laboratory analysis, with the aim of a median of 8.5%. When approximately 50% of the randomised subjects have an HbA1c above 8.5%, the remaining subjects randomised must have an HbA1c below or equal to 8.5% or when approximately 50% of the subjects randomised have an HbA1c below or equal to 8.5%, the remaining subjects randomised must have an HbA1c above 8.5% - Current treatment for at least 90 calendar days prior to screening with basal insulin plus metformin plus/minus α-glucosidase inhibitors, sulphonylureas, glinides or thiazolidinediones. Subjects should be on a stable dose for at least 60 calendar days prior to screening of: Basal insulin 20-50 units (U)/day (both inclusive) ( Individual fluctuations of plus/minus 5U during the 60 day period prior to the day of screening are acceptable.) on the day of screening in combination with: - Metformin (above or equal to 1500 mg or max tolerated dose) or - Metformin (above or equal to 1500 mg or max tolerated dose) and sulphonylureas (above or equal to half of the max approved dose according to local label) or - Metformin (above or equal to 1500 mg or max tolerated dose) and glinide (at least half of the max approved dose according to local label) or - Metformin (above or equal to 1500 mg or max tolerated dose) and α-glucosidase inhibitors (AGI) (at least half of the max approved dose according to local label) or - Metformin (above or equal to 1500 mg or max tolerated dose) and thiazolidinediones (at least half of the max approved dose according to local label) - Body mass index (BMI) above or equal to 24 kg/m^2 Exclusion Criteria: Current use of any antidiabetic drug (except for basal insulin, metformin, α-glucosidase inhibitors, sulphonylureas, glinides or thiazolidinediones) or anticipated change in concomitant medication, that in the investigator´s opinion could interfere with glucose level (e.g. systemic corticosteroids) - Treatment with glucagon like peptide -1 receptor agonists, or dipeptidyl-peptidase-4 inhibitors or insulin (except for basal insulin) within 90 days prior to Visit 1 - Impaired liver function defined as alanine aminotransferase above or equal to 2.5 times upper normal range - Impaired renal function defined as serum-creatinine above or equal to 133 μmol/L for males and above or equal to 125 μmol/L for females, or as defined according to local contraindications for metformin Screening calcitonin above or equal to 50 ng/L - Personal or family history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia type 2 (MEN2) - Cardiac disorder defined as: congestive heart failure (NYHA class III-IV), diagnosis of unstable angina pectoris, cerebral stroke and/or myocardial infarction within the last 12 months prior to screening and/or planned coronary, carotid or peripheral artery revascularisation procedures - Severe uncontrolled treated or untreated hypertension (systolic blood pressure above or equal to 180 mm Hg or diastolic blood pressure above or equal to 100 mm Hg) - Proliferative retinopathy or maculopathy (macular oedema) requiring acute treatment - History of pancreatitis (acute or chronic)