Contacts / Address Book Duplicates

When running a 'check for duplicates' enquiry in address book (Contacts 7.1) a window pops up identifying the number of duplicate cards and duplicate entries found, and provides either a cancel or merge option.

How can I check what the duplicates are before I run the merge function?

My address book contains several thousand contacts - and I do have people with the same names - and running a visual check is not likely to identify the duplicates.

Thanks for any pointers.