Aggregation framework query by _id returns duplicates in sharded cluster (orphan documents)

mlaunch --sharded 2 --config 1 --single --binarypath <bin_version>   use duptest db.dup.insert({_id: 2, a: 2, b: 2}) db.dup.insert({_id: 1, a: 1, b: 1}) db.dup.ensureIndex({a: 1}) sh.enableSharding("duptest") sh.shardCollection("duptest.dup", {a: 1})   sh.splitAt("duptest.dup", {a: 2})   # If needed, perform moveChunk. sh.moveChunk("duptest.dup", {a: 2}, "shard02")     # After split the MaxKey collection should be having the a = 2 document, on that shard add the document to have effect of orphan document db.dup.insert({_id: 1, a: 1, b: 12})   # This results in two records while ideally it should be only 1 db.dup.aggregate([{$match: {_id: 1} }])   # This is expected to return 2 by its current design db.dup.count({_id: 1})