import time
from bson import Binary
from pymongo import MongoClient, DESCENDING

bucket = MongoClient('127.0.0.1', 27017)['test']['foo']
bucket.drop()
bucket.update({'timestamp': 0}, {'$addToSet': {'_exists_caps': 'cap15'}}, upsert=True, safe=True, w=1, wtimeout=10)
bucket.create_index([('timestamp', DESCENDING)], unique=True)
while True:
    timestamp =  str(int(1000000 * time.time()))
    bucket.update({'timestamp': timestamp}, {'$addToSet': {'_exists_foos': 'fooxxxxx'}}, upsert=True, safe=True, w=1, wtimeout=10)

Traceback (most recent call last):
  File "test_mongo_update.py", line 11, in <module>
    bucket.update({'timestamp': timestamp}, {'$addToSet': {'_exists_foos': 'fooxxxxx'}}, upsert=True, safe=True, w=1, wtimeout=10)
  File "build/bdist.linux-x86_64/egg/pymongo/collection.py", line 552, in update
  File "build/bdist.linux-x86_64/egg/pymongo/helpers.py", line 202, in _check_write_command_response
pymongo.errors.DuplicateKeyError: E11000 duplicate key error collection: test.foo index: timestamp_-1 dup key: { : "1439374020348044" }

systemLog:
   destination: file
   logAppend: true
   logRotate: reopen
   path: /opt/lib/log/mongod.log

# Where and how to store data.
storage:
   dbPath: /opt/lib/mongo
   journal:
     enabled: true

   engine: "wiredTiger"
   directoryPerDB: true

# how the process runs
processManagement:
   fork: true  # fork and run in background
   pidFilePath: /opt/lib/mongo/mongod.pid

# network interfaces
net:
   port: 27017
   bindIp: 0.0.0.0  # Listen to local interface only, comment to listen on all interfaces.

setParameter:
   enableLocalhostAuthBypass: false
Any thoughts on what could be going wrong here?