Click Through Error - An item with the same key has already been added - When/Why?

I am getting this error when clicking-through in Report Builder on a report using a report model as the data source -
An item with the same key has already been added.

I searched for the error but came up empty in the SSRS space. Does anyone out there know when this is generated and where I need to look in my model/properties to see what I have set incorrectly?

A little background/detail - It's a simple report against two fields (type & #count) in one entity (customer) in a (guessing here) medium sized model (~16 transactional/fact entities, ~40 lookup entities, ~15 what I am calling domain entities). The entity has 5 DefaultDetailAttributes, one Identifying. The report has 2 fields type (a lookup role) and #customers in the data area (so I can click it to test the click through). Then I click the # Customers to look at the detail for those X customers and after a long while and no queries against the DB (I have profiler running) I get this error.

Thanks in advance for the help.

Getting the same error too...