A few more pieces around the edges of Mongo joins.

I think this is the wrong abstraction. How about a `Duplicate` task?

```
case class DuplicateTask(source: WorkflowTask)
```

Presumably, `DuplicateTask` would only be inserted in some places, where it's known the subsequent task would overwrite the results of the collection. 

Maybe that's not right either but I feel something is missing here.