Because if you don't use separate keys, only one of the three occurrences of the positive account (21, 22 or 23, the one you choose) in the Groups-Accounts dimension would...

Remember the original fact table:

- Account 1: DatePK 20130901, AccountPK 1, Value 10

- Account 2: DatePK 20130901, AccountPK 2, Value 20

- Account 3: DatePK 20130901, AccountPK 3, Value 30

When slicing...

I could, but it doesn't address the issue: what if the user decides to put Account n adding in 3 groups, but substracting in 1? Again results would be off...

I don't know. Obviously I'm still missing something trying to help you. I've seen duplicates in the sense of repeated names. The city of Columbus exists in...

That would work if this dimension was the only one present in the cube. Imagine I duplicate the values for Account 2 and 3 in the fact table, each with...

Yes, I did, and I face the same problem: in a parent-child dimension a given child can have only one parent. In my case, Account 2 and Account 3 have...

The dimension table is what I showed, there is only one flatted hierarchy, the Groups-Accounts hierarchy:

The Parent-Child table from where this originates would be like this:

As I said, the fact table has more foreign keys, I didn't expose them in my original post for simplicity. Indeed, if there were no other dimensions Koen's solution would...

How could I do that with only one dimension?

You mean this?:

And then join DimID with the dimension ID? I already considered doing this, but it's not possible. In the real fact table there are more foreign keys, the...