What does " union " mean in the world of SD ?
As best as i can recall the "duplicated" were folders on drive B that had a different name or were in a different location. So, SD should know that right ???
Your right, SD did it's job.
It's also good to know that the "copy newer" may not make a "bootable" drive. When that happens is there some procedure to follow to bless that drive to make it bootable ?