This is an existing bug. Upserts to local
database fail with
E11000 duplicate key error index: local.properties.$_id_ dup key: { : null }
exception.
Here's an example:
> use local
switched to db local
> db.properties.update({'url': 'test1'}, {'url2': 'test2'}, upsert=true)
> db.properties.update({'url': 'test1'}, {'url2': 'test2'}, upsert=true)
E11000 duplicate key error index: local.properties.$_id_ dup key: { : null }
> use local2
switched to db local2
> db.properties.update({'url': 'test1'}, {'url2': 'test2'}, upsert=true)
> db.properties.update({'url': 'test1'}, {'url2': 'test2'}, upsert=true)