@KevinN_CSJ - I cannot reproduce the issue with the steps you provided.
Here are the steps I took:
1. Opened a project with a layout, Layout A and 2 maps where one map (Map A) has layer A and another map (Map B) has layers A and B.
2. Opened Layout A (with a map frame pointing to Map A).
3. Exported Layout A 3 times to .png output type.
4. From the Catalog pane, right-clicked Layout A and duplicated it using the Duplicate command from the context menu.
5. Opened the new duplicate layout, Layout A1.
6. Selected the map frame, opened its properties and changed the source map from Map A to Map B.
7. Activated layout A.
I did not see any unexpected legend items in Layout A. Without more information I cannot reproduce your scenario and cannot verify that the fix we made for 3.4 and for 3.3.2 works in your scenario.
Please 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