The yield_geo_near_dedup.js workload tests that documents returned by the geoNear command are deduplicated by their RecordId. It assumes the pair (_id, timesInserted) uniquely determines a RecordId; however, that isn't the case in a sharded system where documents can move between shards. Since mongos cannot return information about which shard the document came from, i.e. to deduplicate by the triplet (shard #, _id, timesInserted), the assertions that the workload makes may fail when the balancer is running concurrently.