How to get distinct values from each column

Hi Community,

Please help me in fixing this issue . I have created a table with 3 different columns and want to get distinct values from each column . I am getting duplicates with the below query . Please check and let me know on how to get distinct values . 

Table =

Var a = SELECTCOLUMNS('Product & Apps Incident Query',"Product ID",('Product & Apps Incident Query'[Product_Id]),"Product Name",('Product & Apps Incident Query'[product_name]),"Support Group",('Product & Apps Incident Query'[Assigned_Group]))

Var b = SELECTCOLUMNS('Product & Apps WO Data',"Product ID",'Product & Apps WO Data'[Product_Id],"Product Name",('Product & Apps WO Data'[Product_Name__2_]),"Support Group",('Product & Apps WO Data'[ASGRP]))

return UNION(a,b)

Thanks,

G Venkatesh