Repeat `_id`s

I seem to be getting multiple repeated `_id`s when the db is opened and closed repeatedly, and each session only inserts one entry. Is this expected?

```
>>> TinyDB('db.json').insert({'x': 1})
>>> TinyDB('db.json').insert({'x': 1})
>>> TinyDB('db.json').all()
[{u'_id': 0, u'x': 1}, {u'_id': 0, u'x': 1}]
```

This is with 1.2.0 from pypi

```
$ pip freeze | grep tinydb
tinydb==1.2.0
```
