Getting message "An item with the same key has already been added" when creating an SSRS report

This error is caused by having fields with the same name in the query the report is built upon.  To rectify the issue, in AX go to the query you created and identify the fields that are duplicated between the datasources in the query.  On the Fields node of each data source set the Dynamic property to No.  Changing this property will allow you to remove fields from the query.  Then delete the fields that overlap.