> use sharded
> db.unique.drop()
true
> db.adminCommand( { shardcollection : "sharded.unique", key:{a:1}})

{ "collectionsharded" : "sharded.unique", "ok" : 1 }

> db.system.indexes.find()
{ "name" : "id", "ns" : "sharded.foo", "key" :

{ "_id" : 1 }

, "v" : 0 }
{ "v" : 1, "key" :

{ "_id" : 1 }

, "ns" : "sharded.unique", "name" : "id" }
{ "v" : 1, "key" :

{ "a" : 1 }

, "ns" : "sharded.unique", "name" : "a_1" }

Sharding the collection succeeds and creates two non-unique indexes, one on _id and one on the shard key. It should fail, saying that a unique index is necessary.