Currently, Balancer::_checkOIDs() produces this error message if duplicate oids are detected:

    log() << "error: 2 machines have " << x << " as oid machine piece " << s.toString() << " and " << oids[x].toString() << endl;

This error message can occur if the config server's shards collection contains unique 'host' strings, but with the same replica set name. The following configuration error could cause this:

mongos> db.shards.find()
{ "_id" : "sh1", "host" : "rs1/localhost:27017,localhost:27018,localhost:27019" }
{ "_id" : "sh2", "host" : "rs1/localhost:27027,localhost:27028,localhost:27029" }

The intended configuration is to use 'rs2' for the second line in this example. Although quite rare, this is a very easy error to overlook because the hostnames and shard names are correct. It might be nice to add logic to detect and print a more robust error message when multiple shards are pointing at the same replica set name.

Note that this check could likely live in MoveChunkCommand and SplitChunkCommand.