We're merging a bunch of Salesforce records at work . We got a list of duplicates from an external tool (Cloudingo), but that didn't give us all the duplicates. So we also wrote some custom Ruby code to pull potential duplicates based on additional criteria. The result was two sets of account IDs, like this (very simplified):
List 1: a,b; d,e; f,g
List 2: a,n; d,e,o; x,y