Managing Duplicate Settings can be done at the Contract Level.
From Main Menu, click on Advertisers -> Contracts -> Contracts.
Click on the Contract Name to open the Contract Window.
From the Contract's Window, Click on Settings -> Duplicates & Returns.
Duplicate Leads: When checked this setting will not allow duplicate leads to be posted to this contract.
Duplicates are within: This setting allows you to adjust how far back the duplicate check for this contract will compare data.