Hi,

We have a sharded collection with hashed index on "_id" as the key. We started with 2 shards and added one more.

However we are finding duplicate objects with the same _id on both shard-0 and and shard-2. We identified this by directly connecting to the primary of the shards.

A few other observations
1. Running a find from the mongos does not find the duplicate documents since it probably goes to the right shard

Are these duplicates from a failed migration? If so how come mongod does not clean it up?