As a flexible database type, you will need to observe and practice duplicate prevention. If duplicates can be created within your CRM, they will be added as duplicates inside Acoustic Campaign. Ensure that you implement duplicate awareness or prevention within your CRM.
For example, when you are importing CSV files inside Acoustic Campaign, the email field will default as checked to match on. If you clear the check box, you may introduce duplicates. When creating Web Forms inside Acoustic Campaign, Contact Matching will default to match on 'email' that helps you prevent web form submissions from adding duplicates. The default contact matching behavior was new to 16.3, so if you have forms created before the 16.3 release you should check their settings if you believe duplicates are occurring. If you use the Acoustic Campaign API's to create contacts, be sure sync fields are used to update vs. insert when necessary.