Yes, when you add or upload contacts to your email lists, our system automatically looks for duplicate email addresses and removes them. This ensures that each of your contacts lists only contain unique email addresses.
If you send an email to multiple lists at one time, and some contacts belong to all of your contact lists, each contact will only receive the email once.
Duplicate records are also removed when you merge two/multiple lists.
Note: Sharing contact list from a master account to a sub-account will impact the total number of unique contacts. The sub-accounts contacts are individual to that of the master account contacts.
For that matter, the Master account should have a plan that fits all of the contacts in their account and in their sub-accounts.