Mental health comorbidity is higher in those with learning disability; in the absence of a full and robust diagnostic clarification, diagnostic overshadowing is a particular problem. Central to this concept is attribution of symptoms to an existing diagnosis rather than a potential co-morbid condition. Clinicians need to be aware of the dire consequences of diagnostic overshadowing including misattribution, underdiagnosis, misdiagnosis, delayed and or poor treatment, stigmatization and consequently a reduction in the quality of life. This is a case report of a young male with mild LD with longstanding mental health and behavioural problems who was described as having a personality disorder in the community. Facing multiple charges of assault, the court, on medical advice, gave him a hospital order to a medium secure unit for people with learning disabilities where he went through a detailed and systematic diagnostic evaluation. This revealed several new findings including the diagnosis of a 22q11.2 Duplication Syndrome. In this case, physical impairments associated with 22q11.2 Duplication Syndrome affected patient’s presentation and were wrongly formulated as representative of a Personality Disorder. The atypical autism, learning disability and co-existing mental illness further complicated the picture. Positively, the genetic test and the identification of a syndrome provided valuable clinical insight and significantly altered the nature, course, and pathway of treatment, including ultimately the patient’s quality of life. This resulted in a successful discharge back to the community from a secure hospital setting. In addition, this case report highlights previously unreported findings: Cochlear Nerve Atresia, Tubular Vision, a characteristic groove, and skin fold on the back of the scalp and the presence of a schizoaffective mental illness.