A list do not allow duplicate emails. So if you import a .csv into an existing list and emails are already in the list, the information will be updated with the information from the latest import. The email won't be appear twice.
If you have multiple lists, the same email can be found in more than one list. Please note that if a contact is active in, for example 5 lists, it will count as 5 contacts within your total contacts (linked to your plan). That is why we recommend using, as much as possible, 1 list per language: this helps having the information from a contact (history, campaign received, click, etc) at one place and to avoid paying for the same contact that is present in more than a list. Segmentation in your list can help avoiding this. You can read more about how to segment here.