public IEnumerable <string> GetLearnerDuplicateLearnRefNumbers(List <MessageLearner> learners) { var duplicates = learners.GroupBy(x => x.LearnRefNumber, StringComparer.OrdinalIgnoreCase) .Where(g => g.Count() > 1) .Select(g => g.Key); foreach (var refNumber in duplicates) { _validationErrorHandler.CrossRecordValidationErrorHandler($"Duplicate LearnRefNumber:{refNumber} found for Learners", learners.FirstOrDefault().SourceFileName); } return(duplicates); }