Prevent Duplicate inserts in SQL Server Stored Procedure

hI

this is  Subset_menu table in database

in InsertP.aspx page when users click button ,in above table in subset column insert their data

here I want if they want insert duplicate data in Subset column it don't insert any thing

I mean here if users want insert X123 it didn't insert it in Subset_menu  table becuse there is this data  in subset column

so I worte below SP but it didn't work

SP

USE [behtop]
GO
/****** Object:  StoredProcedure [dbo].[insertproduct2]    Script Date: 03/01/2013 12:38:09 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
 
ALTER procedure [dbo].[insertproduct2]
@Name nvarchar(30),
@Model nvarchar(30)
,@Behcode nvarchar(10)
,@price nvarchar(30)
,@Classification  nvarchar(30)
,@Description nvarchar(max)
,@subset nvarchar(30)
,@id int =0
 
 
 
AS BEGIN
Declare @H_name nvarchar(30) set @H_name =(select H_name from House_info where behcode=@Behcode )
IF @id > 0 AND EXISTS(SELECT behcode FROM House_p WHERE iD = @id)
 
BEGIN
UPDATE House_p
SET Name=@Name
,Model=@Model
,Description=@Description
,Price=@price
,Classification=@Classification
,subset=@subset
,Date=GETDATE()
,H_name=@H_name
WHERE BehCode=@Behcode and ID=@id
 
IF NOT EXISTS(SELECT Subset FROM Subset_menu)
begin
insert into Subset_menu(H_name,Subset)
Values(@H_name,@subset)
end
SELECT @id
END
 
 
ELSE
BEGIN
INSERT INTO House_p(
Name,Model,Price,Classification,subset,[Date],H_name,BehCode,search)
VALUES(
@Name,@Model,@price,@Classification ,@subset ,GETDATE(),@H_name,@Behcode,@Name+' '+@Model)
IF NOT EXISTS(SELECT Subset FROM Subset_menu)
begin
insert into Subset_menu(H_name,Subset)
Values(@H_name,@subset)
end
 
 
SELECT SCOPE_IDENTITY()
END
END