Issues with Deduplication

I have just installed eM client, set up my two email accounts and the system apparently populated the various folders from the two servers, one Gmail and one Plusnet.  I then imported the .pst file into a set of Local folders which of course means I now have a significant number of duplicates between the two account folders and the Local folder.
I have been unable to use the deduplication tool the sort this out, can anyone suggest why?

I have just discovered that the two copies of the same message have different times of receipt, how? I do not know but that must be the reason for the non deduplication