Validate Order

Hi folks, looking for some help, my store proc saves customer order details to ms sql database and is fired by the user clicking on a command button.

I need the stored proc to validate if the OrderID already exists within the database before saving the order, not really sure how to do this. Code below,

USE [TyreSanner]
GO
/****** Object:  StoredProcedure [dbo].[AddCustomerDetails]    Script Date: 11/27/2014 22:32:35 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[AddCustomerDetails]
 
/***********************Declare variables ***********************/
 
/******tblCustomer******/
 @Forename nvarchar(50), 
 @Surname nvarchar(50),
 @HouseNo  nvarchar(50),
 @CustAddress nvarchar(50),
 @Town nvarchar(50),
 @Postcode nvarchar(50),
 @ContactNo nvarchar(50),
 @EmailAddress nvarchar(50),
  
 /******tblLink_OrderProduct******/
 @ProductQuantity int, 
 @TotalProductSaleCost decimal,
 @ProductFK int,
 @FittingDate date,
 @FittingTime Time
As
 DECLARE @CustomerFK int;
 DECLARE @OrderFK int;
 Begin TRANSACTION
 SET NOCOUNT ON 
 
INSERT INTO [TyreSanner].[dbo].[Customer](Forename, Surname, HouseNo, CustAddress, Town, Postcode, ContactNo, EmailAddress)
    VALUES     (@Forename,@Surname,@HouseNo,@CustAddress,@Town,@Postcode,@ContactNo,@EmailAddress)
    
    
    Set @CustomerFK = SCOPE_IDENTITY()
    
   INSERT INTO [TyreSanner].[dbo].[Order]  (CustomerFK)
        VALUES (@CustomerFK)
    
    
  SET @OrderFK = SCOPE_IDENTITY()
  
 INSERT INTO [TyreSanner].[dbo].[Link_OrderProduct](OrderFK, ProductFK, ProductQuantity, TotalProductSaleCost, FittingDate, FittingTime)
     
  VALUES 
 (@OrderFK,  @ProductFK,  @ProductQuantity, @TotalProductSaleCost, @FittingDate, @FittingTime)
 
SELECT SCOPE_IDENTITY() AS Link_Id;
  
 
  
 COMMIT TRANSACTION