We should avoid creating duplicate records. It isn't just a headache for staff, it also results in poor customer experience with CII. For example, a person could end up with 2 profiles, one that gives access to her committee's Groups page and the other that tracked her attendance at a workshop.
Duplicates can be avoided by following a few simple steps before creating a new record.
Sometimes however, duplicates happen, despite our best intentions. This page also covers what to do when that happens.