The claim that we don't want to enforce uniqueness in compact is made here:
https://github.com/mongodb/mongo/blob/v2.6/src/mongo/db/structure/collection_compact.cpp#L157

It doesn't actually work because we go directly to the index descriptor to check if dups are allowed here:
https://github.com/mongodb/mongo/blob/v2.6/src/mongo/db/index/btree_based_access_method.cpp#L413