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.