Organiser - find duplicates and images with a single specified tag only.

Most of my  (25k) image files now have event, people and/or keyword tags, but I still have a lot which are duplicated - images of a which a copy has been saved under a different name and/or in a different folder. Often this has been to send smaller copies to friends or printing services. As a result I still have a lot which are tagged ONLY as "possible duplicates", as well as many having this in addition to other tags. 

Firstly, am I correct in thinking that if I simply delete the "possible duplicates" keyword tag, and then find untagged items (ctrl shift Q) I will identify the ones I haven't yet given any other tags to, without affecting the tags on all the rest?   

Then ideally I would like to be able to check if they ARE in fact true duplicates (e.g. same file size etc) or compressed versions, and make a decision as to deleting them.  But at present I don't see a way to do this....