shardCollection takes a unique parameter that when true requires there is an index supporting that shard key that also guarantees uniqueness for the shard key field. After SERVER-6491, we don't allow dropping the last index supporting the shard key, but we don't consider if the collection was sharded with unique:true. This lets a unique:true sharded collection be supported by a non-unique index if the unique index supporting it is dropped but there is another non-unique index supporting the shard key fields.