How To Force Unique Entries In A Linking Table?

I have a table 'Group2Operation' that stores many to many relations
between the 'Group' table and the 'Operation' table (each group is has
permission to perform one or more of the available operations)

PROBLEM
=======
I need to prevent duplicate entries being created. e.g. lets say that
in the 'Group2Operation' table a record links the 'editor' group to
the 'publish' operation. Should I prevent an administrator creating a
duplicate of that record? (Otherwise deleting that permission will
have to be done twice or more for it to be effective)

SOLUTION?
=========
So far I've done this with a trigger:

CREATE TRIGGER Group2OperationDuplicates ON dbo.Group2Operation
FOR INSERT, UPDATE
AS UPDATE Group2Operation
SET NoDuplicate = CONVERT(nvarchar(10),GroupID) + OperationTag

The 'NoDuplicate' unique index column in the Group2Operation table
stores a concatenation of the unique group and operation identifiers.
So when an attempt is made to create a record, the trigger is fired.
If there is a duplicate, this will mean a duplicate entry in the
'NoDuplicate' column. As a result, the INSERT or UPDATE will fail and
the duplication will be prevented.

WHAT DO YOU THINK?
==================
What do you think? Am I going about this in the right way? Is a
trigger a good way to do this or should I rely on application logic to
prevent duplicates?

Any help appreciated by this db novice.
John Grist