It is possible to send to an email address multiple times, so you need to take this into consideration if you are using a multiple database approach.
In many instances this is a perfectly valid scenario - such as sending newsletters for different subscriptions that have no direct correlation with one another.
However, if you are planning to send a single email template to different contact databases that could potentially contain the same recipient, then you might want to enable frequency control to limit how many emails can be sent to a single recipient each day.
Alternatively, you can create a suppression list created from your very first send and attach to the other emails at send time.
A recommended practice would be to use a single main database and then use contact lists for the purpose of emails sends and if at all possible and if relevant, dynamic content blocks.
If you are using a single database that uses email as the primary key, then only one instance of that recipient exists. If however you are using a flexible database (either another field or none at all in the case of a CRM database), then Acoustic Campaign suppresses all duplicate sends of the template to an email address automatically. You will see a reason code of 11 in the export of the suppressed email report which means: Duplicate Email in Non-Key List.