The significance of this project is to investigate the effects of adjunctive metformin therapy in children and adolescents with double diabetes. Double diabetes describes a clinical state where an individual possesses features of both type 1 and type 2 diabetes. There is a paucity of data on the role of adjunctive metformin therapy in children and adolescents with double diabetes. To help fill this knowledge gap, the investigators propose a randomized, double-blind, placebo-controlled trial of metformin in double diabetes. Specifically, the investigators will evaluate changes in hemoglobin A1c and anthropometry in patients with a diagnosis of type 1 diabetes who also have features of type 2 diabetes or metabolic syndrome as well as patients with type 2 diabetes who possess diabetes-associated autoantibodies. This will help determine the safety profile, and efficacy of adjunctive metformin therapy in these subjects.