Union Transformation in general does not allow duplicate rows. If the sources contain duplicate rows then the effective count of rows after union certainly will be reduced. That should be one of the reasons for it being an active transformation.

A question may be raised regarding a situation where the sources do not contain any duplicates. The answer should be even in that scenario the transformation's inherent ability to check for the duplicate rows will remain intact. Just like a filter transformation not changing the row count because the filter condition was not either entirely true or entirely false for all of the rows.