While processing a dimension, processing got failed and shown the below warning as well.
Errors in the OLAP storage engine: A duplicate attribute key has been found when processing:..
But when I checked the values of the mentioned attribute, there were no duplicate values. But the relevant column data type is INT and it has a value as NULL.
When I checked the dimension attribute key column properties, NullProcessing property is set to Automatic.
Actually this is causing the above error message since during the processing, NULL values are converted to 0 and we already have an attribute value as 0.
Same issue can happen for the string data types having blank value as well.
To fix this we can replace NULL values with the relevant value in the Data Source View of the Cube or replace it in the Data Warehouse using the ETL. Best way to do is to change it in the Data Warehouse.
Below link contains more details about this issue as well.