It is not possible to have duplicate email addresses in the same list. When importing, any duplicates are automatically removed.

However, a single subscriber can be on multiple lists, allowing them to have multiple subscriptions. For instance, they may choose to subscribe to emails sent to a specific list for 'Brand X' but not for 'Brand Y.'

If a subscriber is on multiple lists, they will be counted multiple times for billing purposes. To save costs, using Tags and Fields is a recommended option.