Error exporting subworkflow

When exporting the big all-in-one subworkflow that includes the 2 variants for single-end and paired-end workflows the export fails with
```
galaxy.web.framework.decorators ERROR 2020-06-01 18:04:18,839 Uncaught exception in exposed API method:
Traceback (most recent call last):
File "/Users/mvandenb/src/galaxy/lib/galaxy/web/framework/decorators.py", line 295, in decorator
rval = func(self, trans, *args, **kwargs)
File "/Users/mvandenb/src/galaxy/lib/galaxy/webapps/galaxy/api/workflows.py", line 471, in workflow_dict
ret_dict = self.workflow_contents_manager.workflow_to_dict(trans, stored_workflow, style=style, version=version)
File "/Users/mvandenb/src/galaxy/lib/galaxy/managers/workflows.py", line 480, in workflow_to_dict
wf_dict = to_format_2(wf_dict)
File "/Users/mvandenb/src/galaxy/lib/galaxy/managers/workflows.py", line 461, in to_format_2
return from_galaxy_native(wf_dict, None, **kwds)
File "/Users/mvandenb/src/galaxy/.venv/lib/python3.8/site-packages/gxformat2/export.py", line 54, in from_galaxy_native
output_id = labels.ensure_new_output_label(workflow_output.get("label"))
File "/Users/mvandenb/src/galaxy/.venv/lib/python3.8/site-packages/gxformat2/_labels.py", line 17, in ensure_new_output_label
assert label not in self.seen_labels
AssertionError
```

I think `assert label not in self.seen_labels` might need to be scoped to individual workflows ?