How to remove Duplicates of records using Informatica?

Lets say you have 2 columns in source table. Do the following in an expression:

A(in)
B(in)
A_Pre(V) = A
B_Pre(V) = B
Comp(V) = IIF( A= A_Pre and B= B_Pre, ‘Y’,'N’)
Basically you need to store the previous values in a variable and compare with incoming data.