I created a "Account Data Extension" with AccountID as primary key, so every customer is just ones in the Data Extension. Now I want to send order related Emails/Journeys. Therefore I filter this Data Extension with Fields of my "Order Data Extension". But what happens when a customer has more than one order? Is this customer than twice in the filtered Data Extension?
It is possible because
Contacts can belong in multiple
Account object records as there is a many-to-many relationship between those two objects. You'll have to run a SQL query if you want unique values or you can set the primary key on the Email Address and re-run the filter but that gives you less control on what records you will be selecting to be the unique value.