So when I open up a SQL Server Analysis Services Project in Visual Studio and it gives me the warning "Do not ignore duplicate key errors. Change the KeyDuplicate property of the error configuration so that it is not set to IgnoreError." I thought to myself: "That's a good idea." I ought to do that. A duplicate key is never a good thing...
Um, so how exactly do I do that? I tried changing it when I processed the dimensions (and the cube). And when the warning still persisted I went ahead and tried the same thing again and again and again. Surprisingly, it still didn't work.
Luckily, I have a coworker who was able to point me in the right direction.
First, open the dimension (not the cube) in Visual Studio's Solution Explorer (in my example below the dimension is named "Subject"):
Next, right click on the dimension in the attributes window and select "Properties".
When the Properties window opens find the ErrorConfiguration property and select the dropdown.
Set the ErrorConfiguration to "(custom)" and then expand the "ErrorConfiguration" property. Update the "KeyDuplicate" property to either "ReportAndContinue" or "ReportAndStop".
Hit save and build the project and ta-da! Problem solved!