Explanation
Journey Builder is a tool that allows marketers to create personalized, cross-channel customer journeys based on real-time behavior and data. Journey Builder can use a Recurring Entry Source to inject contacts from multiple data extensions into a journey on a daily basis. Journey Builder can also use Messaging and Flow Control Activities to send emails and mobile messages and control the timing and frequency of the communications.

Explanation
marking the field as the Primary Key is what a marketer should do to ensure the Email Address field values are unique in a data extension. A primary key is a field or a combination of fields that uniquely identifies each row in a data extension. If a field is marked as a primary key, it cannot have duplicate values or null values.
Marking the data extension as Sendable or using Email Address as Subscriber Key does not ensure uniqueness of field values.