There should be option of identifying the duplicate contacts in your address book which were not merged during import. It can be on basis of first name, email id or phone no.
Thank you for writing in and submitting this feature.
Unfortunately,we must decline this feature request. Our current merge algorithm seems to eliminate the chance of duplicate contacts appearing in Nimble since this feature was first submitted. In light of our merge improvements, we have decided that a duplicate identifier will be unnecessary for Nimble.
Thank you for your feedback and support for Nimble!
Dennis van der Heijden commented
I now use this Undupe app... and works really well (I push around 100 duplicates in the system a day)... and cleans it up nicely http://www.undupe.com
Eli kramer commented
Email seems like it would be the most accurate. Or some rough combination of name/company.