Is there any other mechanism that can be exploited in SQL 2008 (not R2 specifically) that could be used to prevent dupes? I regularly use a script to add missing indexes and would like to "lock down" the index creation to eliminate unintentional index and statistic dupes from tuning and manual operations by the DBA and developer teams?
Thanks! You folks are my SQL heroes!

Right. Thanks Paul. That's something I forgot about until I realized that my rowcounts for sys.index_columns was twice what I'd expected.

Naturally I tested on an empty table. The overhead of creating and rolling back the index might not be acceptable on large tables.

The problem with using a DDL trigger is that it is an AFTER trigger, not an INSTEAD OF trigger. So although a trigger would 'prevent' this, the index would still be created, and then it would be rolled-back.

If one wishes to disallow this, a database trigger would do the trick:
[code]
CREATE TRIGGER test ON DATABASE
FOR CREATE_INDEX
AS

SET NOCOUNT ON;

DECLARE @xmlEventData XML
SET @xmlEventData = eventdata()

Declare @table sysname
Declare @indexColumns table (col sysname)
Declare @cmdText varchar(512)
Declare @tempStr varchar(512)
Declare @newIxColCount int
Declare @oldIxColCount int

select @table = Convert(sysname, @xmlEventData.query('data(/EVENT_INSTANCE/TargetObjectName)'))
select @cmdText = Convert(varchar(512), @xmlEventData.query('data(/EVENT_INSTANCE/TSQLCommand)'))

--print @cmdText

SELECT @tempStr =
LTRIM(RTRIM(REPLACE(REPLACE(
SUBSTRING(@cmdText,
charindex(@table, @cmdText) + len(@table),
charindex(')', @cmdText, charindex(@table, @cmdText) + len(@table))
),
'(', ''),
')', '')
));

WITH indexColumns(n, start, stop) AS (
SELECT 1, 1, CHARINDEX(',', @tempStr)
UNION ALL
SELECT n + 1, stop + 1, CHARINDEX(',', @tempStr, stop + 1)
FROM indexColumns
WHERE stop > 0
)
INSERT @indexColumns
SELECT LTRIM(RTRIM(SUBSTRING(@tempStr, start, CASE WHEN stop > 0 THEN stop-start ELSE 512 END))) FROM indexColumns

SELECT @newIxColCount = @@ROWCOUNT

--select * from sys.columns WHERE object_id = object_id(@table)
--SELECT * FROM sys.index_columns WHERE object_id = object_id(@table)
--select * from @indexColumns

SELECT @oldIxColCount = count(*)
FROM sys.columns c
JOIN sys.index_columns ic
On c.column_id = ic.column_id
And c.object_id = ic.object_id
FULL OUTER JOIN
@indexColumns new
On new.col = c.name
WHERE c.object_id = object_id(@table);


IF @newIxColCount * 2 = @oldIxColCount
BEGIN
Declare @err varchar(312)
SET @err = 'Index covering exact columns on [' + @table + '] already exists. Index creation failed'
RaisError (@err, 16, 1)
Rollback
END

--print @oldIxColCount
--print @newIxColCount
GO
[/code]

Perhaps adding a warning would be an improvement.