When sending an Email or SMS, Which50 by default will automatically exclude any duplicate contacts found in your Contact List, however you can ignore this setting if necessary.
And now you can select which field should be used to determine the Duplicate Contacts, with the "Allow Duplicates for Field" drop-down menu. Previously this feature would have been restricted to duplicate email addresses, or duplicate mobile numbers, but now it can be used in conjunction with any Field, for example duplicate Account Numbers.