Allow identical joins only once

[Jud]

We have multiple loosely coupled methods that pass around unexecuted Collection objects and join/filter as they need. It's possible to get into a situation where multiple methods need to join a table, but can't be certain whether it has or has not been joined.

This only allows distinct join statements -- which makes it easier to have more loosely coupled methods that pass around Collections.

[bjornpost]

LGTM :+1:

[bjornpost]

Can you rebase this?

[bjornpost]

@Jud can you rebase this to master?