Which version of Actuate are you using? Are you using the Graphical Query builder? You can use the "Distinct" option from the SQL menu option.
If you can't do it this way, then you will need to override the data source's Fetch method, creating 2 new data row variables: one will need to be static, so that you can compare the previous row to the current row.
Once you can give some more details, I can provide some code if you need (for the Fetch).
Bill