Prevent (Avoid) duplicate record update using Dapper in VB.Net

Dear sir,

I'm trying to update sql to database for fields (CatProd) without duplicates with dapper in VB.NET

in the code below there is method `UpdateCatProd` but how can I do an update sql for the catprod field itself and without duplicates as well 

Please Guide me

Imports System.ComponentModel
Imports System.Data.OleDb
Imports Dapper

Public Class Form1
    Dim CatProdService As New CatProdService()
    Private bindingSource As BindingSource = Nothing
    Private selectedOrder As String
    Private selectedIndex As Integer
    Private Sub Btnsave_Click(sender As Object, e As EventArgs) Handles Btnsave.Click
        Dim id As Integer
        Integer.TryParse(btxtid.Text, id)
        Dim itemid = CatProdService.GetCatProdById(id)
        Dim item = CatProdService.GetByCatProd(txtCatProd.Text)

        Try
            If txtCatProd.TextLength > 3 Then
                MessageBox.Show("CatProd Product Only 3 Characters or Digits")
                Return
            End If
            If itemid IsNot Nothing Then
                If String.IsNullOrEmpty(txtCatProd.Text) OrElse String.IsNullOrEmpty(txtDesCatProd.Text) Then
                    MsgBox("Required fill CatProd,DesCatProd", MsgBoxStyle.Information, "Information")
                    Return
                End If

                If item IsNot Nothing Then
                    MessageBox.Show("CatProd already exists")
                    Return
                End If

                'insert new record
                CatProdService.InsertCatProd(New CatProd() With {
            .CatProd = txtCatProd.Text,
            .DesCatProd = txtDesCatProd.Text
        })
            Else
                'Update record
                CatProdService.UpdateCatProd(New CatProd() With {
            .DesCatProd = txtDesCatProd.Text,
             .CatProd = txtCatProd.Text
             })

            End If
            MessageBox.Show("Successfull")

            LoadData1()
        Catch ex As Exception
            MessageBox.Show(ex.Message, "TEST", MessageBoxButtons.OK, MessageBoxIcon.Exclamation)

        End Try
    End Sub

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        LoadData1()
    End Sub
    Private Sub LoadData1()
        Dim CatProd = CatProdService.Getitem()
        bindingSource = New BindingSource With {.DataSource = New BindingList(Of CatProd)(CType(CatProdService.Getitem(), IList(Of CatProd)))}
        DataGridView1.DataSource = bindingSource
        DataGridView1.ReadOnly = True
    End Sub
    Private Function GetSELECT() As CatProd
        Return If(DataGridView1.SelectedCells.Count = 0, Nothing, TryCast(DataGridView1.SelectedCells(0).OwningRow.DataBoundItem, CatProd))
    End Function
    Private Sub DataGridView1_CellContentClick(sender As Object, e As DataGridViewCellEventArgs) Handles DataGridView1.CellContentClick
        Dim dgv = DirectCast(sender, DataGridView)
        If e.RowIndex >= 0 Then
            Dim SelectDgv = GetSELECT()
            txtCatProd.Text = SelectDgv.CatProd
            txtDesCatProd.Text = SelectDgv.DesCatProd
            btxtid.Text = SelectDgv.ID
        End If
    End Sub
End Class
Public Class CatProd
    Public Property ID() As Integer
    Public Property CatProd() As String
    Public Property DesCatProd() As String
End Class
Public Class CatProdService

    Public Function GetOledbConnectionString() As String
        Return "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\CatProd.accdb;Persist Security Info=False;"
    End Function
    Private ReadOnly _conn As OleDbConnection
    Private _connectionString As String = GetOledbConnectionString()
    Public Sub New()
        _conn = New OleDbConnection(_connectionString)
    End Sub
    Public Sub InsertCatProd(ByVal Obj As CatProd)
        Dim sql = $"INSERT INTO `CatProd` (`CatProd`,`DesCatProd`) VALUES ('{Obj.CatProd}','{Obj.DesCatProd}');"
        Using _conn = New OleDbConnection(GetOledbConnectionString())
            _conn.Execute(sql)
        End Using
    End Sub
    Public Sub UpdateCatProd(ByVal Obj As CatProd)
        Dim sql = $"UPDATE `CatProd` Set `DesCatProd` = '{Obj.DesCatProd}' WHERE `CatProd`='{Obj.CatProd}';"
        Using _conn = New OleDbConnection(GetOledbConnectionString())
            _conn.Execute(sql)
        End Using
    End Sub
    Public Function Getitem() As IEnumerable(Of CatProd)
        Dim sql = "SELECT * FROM CatProd"
        Using _conn = New OleDbConnection(GetOledbConnectionString())
            Return _conn.Query(Of CatProd)(sql).ToList()
        End Using
    End Function
    Public Function GetByCatProd(ByVal CatProd As String) As CatProd
        Dim sql = $"SELECT * FROM CatProd WHERE CatProd = '{CatProd}'"
        Using _conn = New OleDbConnection(GetOledbConnectionString())
            Return _conn.Query(Of CatProd)(sql).FirstOrDefault()
        End Using
    End Function
     Public Function GetCatProdById(ByVal CatProdNo As Integer) As CatProd
        Dim sql = $"SELECT  * FROM CatProd WHERE id = {CatProdNo}"
        Using _conn = New OleDbConnection(GetOledbConnectionString())
            Return _conn.Query(Of CatProd)(sql).FirstOrDefault()
        End Using
    End Function
End Class

Link ms access database