Prevent duplicate value to insert into a table

Dear Sir,
I have written a INSERT Trigger to insert records from Table1(Col1,Col2) to Table2 (Col1,Col2). I want to prevent duplicate value to insert into the column Col1 in the Table2. How Can I do this ?
Thanks.

Use DISTINCT, and use NOT EXISTS correlated subquery. Search Books Online for the

I want to do this thru Code. I want to check that a.no_of_item is already exist in the column no_of_item of the pur_gi_header table.

Original Code given below
CREATE trigger [sub_contracting_order]
on
dbo.pur_gi_header
for insert
as

BEGIN
If EXISTS(select 'X' from inserted a where a.po_category = 'SER' AND a.no_of_item = '0' )

BEGIN
EXEC pur_msg_rtr '02999200052', 'For Service Orders, Machine No is Mandatory' , 'I'

END
END