Right now, there are several places in the code that check whether an index with "clustered" is valid (here and here). By the time we get to _doesSpecClonflictWithExisting, _isSpecOK should already have confirmed a spec with "clustered" is valid with the collection specifications (and thus the clustered index specifications if the collection is clustered)

Additionally, IndexCatalogImpl::_doesSpecConflictWithExisting expects IndexAlreadyExists to be returned for indexes that already exist. Make this consistent for a clustered index