Calling addShard multiple times results in:

> db.runCommand(

{listshards:1}

)
{
"shards" : [

{ "_id" : "shard0", "host" : "localhost:10000" }

,

{ "_id" : "shard1", "host" : "localhost:10000" }

,

{ "_id" : "shard2", "host" : "localhost:10001" }

],
"ok" : 1
}

...which I don't think makes a lot of sense.