Errors in the OLAP storage engine: A duplicate attribute key has been found when processing

This is my 100th post and hence decided to go with the most useful stuff in this post. n this post we are gonna discuss about the most frequently occurred error in SSAS. The error message looks like below (table and column names may vary).

Errors in the OLAP storage engine: A duplicate attribute key has been found when processing: Table: 'dbo_DimProduct_test', Column: 'ProductKey', Value: '227'.