Although duplicate records are a potential patient safety hazard, the actual clinical harm associated with these records has never been studied. We hypothesized that duplicate records will be associated with missed abnormal laboratory results.A retrospective, matched, cohort study of 904 events of abnormal laboratory result (HgbA1c, TSH, Vitamin B(12), LDL). We compared the rates of missed laboratory results between patients with duplicate and non-duplicate records from the ambulatory clinics. Cases were matched according to test and ordering physician.Duplicate records were associated with a higher rate of missed laboratory results (OR=1.44, 95% CI 1.1-1.9). Other factors associated with missed lab results were tests performed as screening (OR=2.22, 95% CI 1.4-3.4), and older age (OR=1.15 for every decade, 95% CI 1.01-1.2). In most cases test results were reported into the main patient record.Duplicate records were associated with a higher risk of missing important laboratory results.