Artifact API Import Error for Series of Sequences

[thermokarst]

Hey @jwdebelius!

This part of the first traceback jumped out at me:

@jwdebelius

    136                 raise ValueError("Transformer from %r to %r already exists."
--> 137                                  % transformer_record)

That bit of the framework checks for duplicated transformers, and according to that it found one... What plugins do you have in this env?