I have a mongodb server with four shards.
first, I create a collection 'PERSON', then I specify a shard key '_id' by following code:

 MongoDatabase adminDB = client.getDatabase("admin");
  try {
      adminDB.runCommand(new Document("enablesharding", "myDB"));
  }catch(Exception e) {
      System.out.println(e.getMessage());
  }
  MongoDatabase db = client.getDatabase("myDB");
  db.createCollection("PERSON");
  adminDB.runCommand(new Document().append("shardCollection","myDB.PERSON").append("key",new Document().append("_id","hashed")));

finally , I saved some data to the collection. but when I try to query person info with some conditions, I got two records with total same values include ids. I logged into each shard and checked my record. I found the record with same id appeared in both shard0 and shard3.

btw：
My data is saved to mongodb through spark conector( artifactId: mongo-spark-connector_2.11, version: 2.1.0) using data set api.