Lead lists arrive from everywhere - form fills, purchased lists, event scans, and partner shares - and they overlap heavily. Importing duplicates inflates your database, double-counts conversions, wastes send credits, and gets the same prospect emailed by two campaigns. A disciplined dedupe pass before import keeps costs and metrics honest.
This workflow merges lead sources, removes duplicate emails and lines, and finds the overlap between lists so you know exactly what's net-new before anything hits your CRM or ESP.