In a properly sorted resident-load you could use interecord-functions like Peek() or Previous() ? and there you could use an expression like:
-rangemin([Emp_No.]<>previous([Emp_No.]), [Date]<>previous([Date]), Amt<>previous(Amt)) as DuplicateFlag
If your load-times are further too slow you will need an incremental load-approach. Within the last two link-blocks in this document you will find various informations about this topic: Advanced topics for creating a qlik datamodel.