how to map to ignore duplicates

Hi all,

My interface is ( EDI 810 formate) file  to Idoc Scenario.

In this my input is having some duplicate records in one file.we have to find the duplicate and we will ignore that duplicate record.

How to change  the mapping ??

1. 1. Invoice number
2. 2. Invoice date
3. 3. Amount
4. 4.vendor number 

plz any one give the idea to find duplicates

Thanks in advance.

regards,

Raj