Deduplication eliminates duplicate copies of data. This is used in email marketing when importing, or gathering, new data to ensure that people who are already on your contact list are not added again.
Why do I need to dedupe?
Though quite a simple action, deduplication helps prevent sending multiple campaigns to the same contact, which could lead to that individual unsubscribing. It is also another element of data hygiene: making sure you hold only one record for each subscriber.
You can dedupe on any of your contact fields but generally using the email address makes the most sense. This is because other fields have a tendency to either change more regularly or to be held by more than one person (such as phone number, names, occupation).