To turn this setting ON, go to Groups and E-mail Addresses and select your Group. Click the "Enable/Disable Duplicate Members" button in the right hand pane.
With this setting enabled, if you attempt to add an address to a Group that already contains that address, it will prevent the new entry. The method of adding a Group member will not affect this setting.
Manual entries will display an error, and imports will skip addresses already in the Group. If you are using a web site sign up form, the user will get an error if they are already in the Group, preventing a second entry for that email address.
Note the difference between this and a Scrub (Scrub Group Members), for a Scrub does not prevent duplicates from being added--it just clears duplicate entries (by email address) when executed.
Note: If you enable this setting on a Group that already has duplicates, the existing duplicates are NOT deleted unless you run a Scrub.