We find there the same _id exist on different sharding, but we set different mode in client to query this _id data ,there will be different results, such as when we use the primary mode query returns only one data

{ "_id" : ObjectId("561fce30fd902e1234426ba7"), "adGroupId" : 6025696187079 }

when we use secendaryPreferred or primaryPreferred mode query returns two data

{ "_id" : ObjectId("561fce30fd902e1234426ba7"), "adGroupId" : 6025696187079 } { "_id" : ObjectId("561fce30fd902e1234426ba7"), "adGroupId" : 6025696187079 }