All,
An update. There is a logged issue - "BUG-000167906 A legend item element gets duplicated when copying a shapefile from a map frame on one layout to another map frame on another layout." Though the title is specific to shapefiles I no longer believe the problem is specific to shapefiles. I've been able to reproduce with file GDB data.
The key to spotting this issue is that the duplication is done on the legend pointing to the map frame/map that you copied the layer from and not legend pointing to the map frame/map you pasted to.
Now that we have a reproducible case we can get our developers debugging the issue. Hopefully the fix is straight forward and one that can be included in a service pack. Of course we need to gather more info before making a call on the service pack.
I want to thank all of you for reporting this issue and providing as much information as you could. Special thanks to those I had the pleasure of meeting at this years UC and troubleshooting this issue in real-time.
Thanks You!!!!
Tom