You need to create an agent to achieve this

HCL Notes/Domino 8.5 Forum (includes Notes Traveler)

Subject:

The Notes/Domino view indexer has a very fundamental limitation in that it can only process one document at a time. I.e., one cannot build views whose selection and/or column formulas depend on a comparison of different documents.

What you could do is write an agent that does something along these lines:

Create a List that takes the Contract ID as the list tag and a Notes document as the value; something like Dim byContractID List As NotesDocument.
Loop through all relevant documents in the database. For each document found, check whether its Contract ID is previously present in the above list. If the answer is no, simply add the document to the list. If the answer is yes, put the current document into a folder (not view). Optionally, put also the document previously in the list under the same Contract ID into the same folder.

After running the agent, you'll find all duplicates (plus optionally their original counterparts) in said folder.

Feedback response number JSAKA44NKE created by ~Tip Zekjumitexakol on 11/09/2015