"Duplicate attribute key" error when attribute is not a key

This is usually a result of having both blanks and NULLs in the source table/view.

Essentially, SSAS does this for every attribute SELECT DISTINCT COALESCE(attr,'') FROM SOURCE

Analysis services by default converts NULLs to blanks, resulting in duplicate value blanks in the resulting feed - hence the error.

I agree this sucks and is a major pain for new players.

Solution : Remove all nulls from the data source, for example by using ISNULL / COALESCE everywhere, or filtering out rows containing null using where clause, or running update statement to replace all nulls with values before processing the cube, etc.

Right click the attribute and select "Properties". Find "KeyColumn" which is located under the "Source" category within the Properties Window. Edit the "KeyColumn" property, it will display a user friendly window.

Remove the attribute from the right (Key Columns) side of the window and replace it with the actual id column from the left (Available Columns) side.

Then edit the "NameColumn" property, the same window will appear. Move the attribute column (the actual data you want to display) from the left side to the right.

Tested in VS 2010 Shell SSDT.

I had the same issue, and there was no blank or NULL values in the attribute. After some analysis, I found that some strings had line break character on the end. So, if 2 values of the attribute are nearly the same, but one of them has line break character at the end, and the other doesn't, then SSAS raises “Duplicate attribute key” error.
It can be fixed by removing line break character from the attribute.
I created calculated column with following definition:

REPLACE(REPLACE(ISNULL([AttributeColumn], ''), CHAR(13), ''), CHAR(10), '')

I used this calculated column in the cube, and the error disappeared.