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);
}