I have created a custom external Id field [Unique Case Sensitive] in a custom object. I have inserted two records having External Id data as 'abc123' and 'ABC123' .  while querying for 'abc123' I am getting both the records. But I want only 1 record whose external Id is 'abc123' How to over come from this Issue.  

My Query : SELECT * from customObject__c where externalSystemId__c = 'abc123' ;  

note: If I do the same in standard object only 1 record comes.