Pluck duplicates first 101 records for query cache

Hi, I use QueryCache in project. When I call .pluck(:id) at first time result duplicates first 101 ids.  
```
2.0.0-p576 :029 > Mongoid::QueryCache.clear_cache
 => nil 
2.0.0-p576 :030 > Mongoid::QueryCache.cache {Unit.where(source: s).pluck(:id).count}
 => 677 
2.0.0-p576 :031 > Mongoid::QueryCache.cache {Unit.where(source: s).pluck(:id).count}
 => 576 
```


 In fact there are 576 records in database.