Hai Pawan Sharma, 

_sql_constraints = [        ('name_uniq', 'UNIQUE (name)',  'You can not have two users with the same name !')    ]

'name_uniq' is constraint name give any name.(no need to define)

Example _constraints:-

    def _check_name(self, cr, uid, ids, context=None):
        for val in self.read(cr, uid, ids, ['name'], context=context):
            if val['name']:
                if len(val['name']) < 6: 
                   return False       

            return True

    _constraints = [        (_check_name, 'Name must have at least 6 characters.', ['name'])    ]

Note: Based on the above example in the records name field any duplicate record or name field less than 6 character then remove the records and add the constraints in python restart the server, update the module then only it works.

 _sql_constraints = [
 ('value_campus_studenttype_policyfor_uniq', 'unique (campus_id,student_type,policy_for)', 'Record for this Campus and Student Type and Policy are already exists !')
 ]

Try this one