How to prevent insert duplicate in SQL Server

Question

I have a stored procedure to insert data into 2 tables. I uses begin and commit transaction. But my if statement before the 2nd insert is not working. Now I have a duplicate entry not only on the first table but as well as on the 2nd table. Below is my stored procedure and hopefully someone can assist me.

USE [DhipayaHQDB_Test]
GO
/****** Object:  StoredProcedure [dbo].[spInsertMotorInsuranceShortTerm_transaction]    Script Date: 8/9/2017 4:19:46 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author:      <Author,,Name>
-- Create date: <Create Date,,
-- Description: <Description,,
-- =============================================
ALTER PROCEDURE [dbo].[spInsertMotorInsuranceShortTerm_transaction]
    -- Add the parameters for the stored procedure here

          (
                @MotorInsuranceID int,
                @RefNumber nvarchar(15),
                @IssuanceType nvarchar(50),
                @CustomerNo nvarchar(50),
                @VehicleType nvarchar(50),
                @VehicleBrand nvarchar(50),
                @VehiclePlateNo nvarchar(50),
                @VehicleEngineNo nvarchar(50),
                @VehicleChasisNo nvarchar(50),
                @VehiclePower nvarchar(50),
                @InsuranceType nvarchar(50),
                @InsuranceOption nvarchar(50),
                @PeriodFrom date,
                @PeriodTo date,
                @ApplicationTime nvarchar(50),
                @DateIssued date,
                @NetPremium decimal(18,2),
                @RegistrationFee decimal(18,2),
                @VAT decimal(18,2),
                @TotalPremium decimal(18,2),
                @BranchCode varchar(50),
                @AgentID nvarchar(50),
                @Username varchar(50),
                --@TransactionNumber nvarchar(50),
                @AgentGrossCommission decimal(18,2),
                @AgentTax decimal(18,2),
                @AgentNetCommission decimal(18,2),
                @ProductType varchar(50),
                @Seller nvarchar(50),
                @Remarks nvarchar(50),

                --customer area
                @CustomerID int,
                @ReferenceNo nvarchar(50),
                @CustomerName nvarchar(255),
                @TaxPayersID nvarchar(50),
                @Village nvarchar(50),
                @District nvarchar(50),
                @Province nvarchar(50),
                @MobilePhone nvarchar(50),
                @CustTelephone nvarchar(50),
                @CustEmail nvarchar(50),
                @Agent nvarchar(50),
                @Status nvarchar(50),
                @TrailDate nvarchar(50),
                @TrailTime nvarchar(50))

AS
BEGIN

    BEGIN TRY

        BEGIN TRANSACTION

            -- insert statement for tblMotorInsurance_eCI

            INSERT INTO [dbo].[tblMotorInsurance_eCI]
                       (
                        [RefNumber]
                       ,[IssuanceType]
                       ,[CustomerNo]
                       ,[VehicleType]
                       ,[VehicleBrand]
                       ,[VehiclePlateNo]
                       ,[VehicleEngineNo]
                       ,[VehicleChasisNo]
                       ,[VehiclePower]
                       ,[InsuranceType]
                       ,[InsuranceOption]
                       ,[PeriodFrom]
                       ,[PeriodTo]
                       ,[ApplicationTime]
                       ,[DateIssued]
                       ,[NetPremium]
                       ,[RegistrationFee]
                       ,[VAT]
                       ,[TotalPremium]
                       ,[BranchCode]
                       ,[AgentID]
                       ,[Username]
                       --,[TransactionNumber]
                       ,[AgentGrossCommission]
                       ,[AgentTax]
                       ,[AgentNetCommission]
                       ,[ProductType]
                       ,[Seller]
                       ,[Remarks]
                       )
                 VALUES
                       (
                         @RefNumber,
                       @IssuanceType,
                       @CustomerNo,
                       @VehicleType,
                       @VehicleBrand,
                       @VehiclePlateNo,
                       @VehicleEngineNo,
                       @VehicleChasisNo,
                       @VehiclePower,
                       @InsuranceType,
                       @InsuranceOption,
                       @PeriodFrom,
                       @PeriodTo,
                       @ApplicationTime,
                       @DateIssued,
                       @NetPremium,
                       @RegistrationFee,
                       @VAT,
                       @TotalPremium,
                       @BranchCode,
                       @AgentID,
                       @Username,
                       --@TransactionNumber,
                       @AgentGrossCommission,
                       @AgentTax,
                       @AgentNetCommission,
                       @ProductType,
                       @Seller,
                       @Remarks
                       )

            -- insert statement for tblcustomers_shortterm

            if exists (select * from tblCustomers_ShortTerm where CustomerNo=@CustomerNo)
            begin
                INSERT INTO [dbo].[tblCustomers_ShortTerm]
                      (
                        [ReferenceNo]
                        ,[CustomerNo]
                        ,[CustomerName]
                        ,[TaxPayersID]
                        ,[Village]
                        ,[District]
                        ,[Province]
                        ,[CustTelephone]
                        ,[MobilePhone]
                        ,[CustEmail]
                        ,[AgentID]
                        ,[Status]
                        ,[Username]
                        ,[TrailDate]
                        ,[TrailTime])
                     VALUES
                     (
                         @ReferenceNo,
                         @CustomerNo,
                         @CustomerName,
                         @TaxPayersID,
                         @Village,
                         @District,
                         @Province,
                         @MobilePhone,
                         @CustTelephone,
                         @CustEmail,
                         @Agent,
                         @Status,
                         @Username,
                         @TrailDate,
                         @TrailTime)
            end

        COMMIT TRANSACTION

    END TRY

    BEGIN CATCH

        ROLLBACK TRANSACTION
        RAISERROR('An error occured. All entries are reverted to it original state.',16,1)

    END CATCH
END

Thanks in advance.