The Duplicate Check option prevents the same product from being ordered again using the same credit card. This is primarily used in trial subscriptions to prevent customers from ordering the same product multiple times with the same payment method. You can configure this in your account settings under Settings > Account settings.
How to Configure Duplicate Checking
Click Go to E-commerce.
Go to Settings in the main navigation.
Click the Account settings button in the organization preview section.
Click the Personal Information tab.
Toggle the Duplicate Check switch to enable or disable.
Click Save to apply your settings.