avoid insert xml duplicate data
Given this table:

CREATE TABLE IQ
(
    XMLDT xml NULL
);


We first need a scalar function to extract the value of interest from the XML:

CREATE FUNCTION dbo.GetRow(@XMLDT AS xml)
RETURNS nvarchar(10)
WITH SCHEMABINDING
AS
BEGIN
    RETURN @XMLDT.value('(./row/text())[1]','nvarchar(10)');
END;


Now we can add a computed column that uses the function:

ALTER TABLE dbo.IQ
ADD RowValue AS dbo.GetRow(XMLDT);


And a constraint to enforce the desired uniqueness:

ALTER TABLE dbo.IQ
ADD CONSTRAINT UQ__IQ_XMLDT_RowValue
UNIQUE (RowValue);


Demo:

-- Succeeds
INSERT INTO IQ (XMLDT)
VALUES('&lt;row&gt;sdsdf&lt;/row&gt;');

-- Msg 2627, Level 14, State 1, Line 1
-- Violation of UNIQUE KEY constraint 'UQ__IQ_XMLDT_RowValue'
INSERT INTO IQ (XMLDT)
VALUES('&lt;row&gt;sdsdf&lt;/row&gt;');