On import (copying from one drive to another) Lightroom is allowing duplicates even when "Don't allow suspected duplicates" is checked. The file name, size,creation date etc. are exactly the same. What I did was import (copy) files from an external drive into Lightroom on my main computer. Then later I wanted to make sure I had imported them all from that drive (I have been traveling a lot and didn't want to lose any images), so I said import (figuring any duplicates would be ignored). It added them all again! I have double checked...the files are identical in every way.