Duplicate activities appear in the workflow and the workflow can't be saved because the activity internal name is not unique.
This is another error arising from editing in two places. User opens the Campaign on its own tab and edits the workflow there. User adds an activity step to the workflow and save it.
User then looks at the campaign workflow in the editing window on the lower right, under the list of campaigns in the Explorer view. Not seeing the activity step he or she adds it again and attempts to save.
The save fails, because an activity with that name is already associated with the workflow. Both activities now show up on the diagram, but the workflow can't be saved.
Create a new workflow and copy and paste the activities (except the phantom!) into the new workflow.
Only edit in one place! Ideally open the Campaign on its own tab and edit it there. Only use the window on the lower right for viewing. Refresh frequently!