Hi @HollyTorpey_LSA
The comment- "When he tried to to turn off one of the layers, it wouldn't let him uncheck it." - sounds like a known issue with resourcing an empty legend to map that has an ArcGIS Map service layer, or WMS layer. This will also duplicate the legend item. I have not seen this behavior with non service layer types.
Since the scenario you outline makes it seem that the issue does not occur until after exporting I would treat your issue as a separate bug. I ran a quick test with 3.3 and did not see any legend item duplication issues with non service layers.
Doesn't mean there is not a bug. Just means I need more information to reproduce.
Can you contact Esri Support? They are better equipped to handle troubleshooting steps and gather bug data. They can be contacted at https://support.esri.com/en/contact-tech-support.
Thanks,
Tom