How does Rule handle duplicates of email addresses?

When importing subscribers to Rule, any duplicates of email addresses are automatically sorted out.

Good to know:

Please note that this may be a reason for your imported subscriber count to differ from your total subscriber count before the import.
Also note that subscribers can of course be registered with different email addresses. Duplicates are, as mentioned, sorted out.
When sending campaigns to multiple tags as recipients, where the same address appears in both tags, the email addresses are automatically merged so that only one sendout occurs.