Add check to prevent the same buffer appearing more than once in a single join

[mxgrey]

Right now it is technically possible for a user to specify the same buffer multiple times within one join operation, but this would result in a Broken error while the workflow is running.

We should instead check whether the same buffer has been put multiple times into the join while building the join instead of risking a runtime problem. We can introduce a new safe_join method that returns a Result, and the existing join method just unwraps safe_join.