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 two parents: Group 3 and Group 5. This gives two duplicate key errors that don't let me process the dimension. If I ignore these errors and process anyway, then Account 2 and Account 3 only appear as children of the first parent, in this case Group 3, just like with the flattened hierarchy.

I've read about ragged hierarchies, and if I'm not mistaken my flattened hierarchy is already ragged. As said in the first post I'm using the HideMemberIf property and it works just fine.