title: find_one() finds duplicates that are not there
body: <p>I am trying to copy a remote mongodb atlas server to a local one. I do this by a python script which also checks if the record is already there. I see that eventhough the local database is empty my script find duplicates, which are not in the remote mongodb atlas (at least i cannot find them). I am not so experienced with mongodb and pymongo but I connot see what I am doing wrong. Sometimes Find_one() finds exactly the same record as before (all the fields are the same even the _id) ? </p>
  
  <p>I removed the collection completely from my local server and tried again, but still the same result.</p>
  
  <pre class="lang-py prettyprint-override"><code>UserscollectionRemote = dbRemote['users']
  UserscollectionNew = dbNew['users']
  
  LogcollectionRemote = dbRemote['events']
  LogcollectionNew = dbNew['events']
  
  UsersOrg = UserscollectionRemote.find()
  
  for document in UsersOrg:   # loop over all users
  
     print(document)
  
     if UserscollectionNew.find_one({'owner_id': document["owner_id"]}) is None:  # check if already there
          UserscollectionNew.insert_one(document)
  
     UserlogsOrg = LogcollectionRemote.find({'owner_id': document["owner_id"]})  # get all logs from this user
  
     for doc in UserlogsOrg:
           try:
               if LogcollectionNew.find_one({'date': doc["date"]}) is None: # there was no entry yet with this date
                   LogcollectionNew.insert_one(doc)
               else:
                   print("duplicate");
                   print (doc);
           except:
               print("an error occured finding the document");
               print(doc);
  </code></pre>