Setting a duplicate value in a column to null or zero

I have a dataflow where I'm joining projects to their respective tasks. Each project will have a large number of tasks associated with it and a planned/actual cost for the project. This results in many duplications of the cost and task data. Can I somehow set all duplications, after the first set, to 0 or null without removing the entire row? The entire row isn't a duplication but many of the columns contain a duplicated value. Its really messing up my graph by duplicating cost.