Does marketing cloud de-dup records based on email or subscriber key?

For Email Studio sends and Automation Studio sends, there is an optional feature to exclude duplicate email addresses. However, this feature is not available for Journey Builder sends, so if exclusion is necessary, duplicates need to be removed beforehand.

On the other hand, Journey Builder has an entry mode that allows controlling re-entry based on the subscriber key. Therefore, it is possible to exclude duplicate subscriber keys within Journey Builder.