Inserting Nonduplicate data

How would can i insert a range of numbers without inserting duplicate value for a certain field? Here is what i have to insert the range.

declare @id int,
@id2 varchar(10)

set @id = 1234
while @i < 4321 begin
set @id2 = 'ID_' + cast(@i as varchar(9))
INSERT INTO tblxxxx (Axxx, Bxxxx, Cxxxxxx)
Values (@id2,1,31,40)
set @i = @i + 1
end

you can always build index on that field and put the Create unique constraint. This way when you'll insert you'll get a error message