How to use multiple command text in a button?

Question

my code:

My Database:

RPRODUCT                PRODUCT
>ID_RPRODUCT         >ID_PRODUCT
>ID_PRODUCT           >NAME
>DATE_IN                >CURRENT_STOCKS
>STOCK_IN


my code is in RPRODUCT form 

=====================

Private Sub btnAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAdd.Click

        Dim cmd, cmd2, cmd3 As New SqlCommand

        Dim connectionString As String = "Data Source=.\SQLEXPRESS;|MyDB|;Integrated Security=True;Connect Timeout=30;User Instance=True"

        con = New SqlConnection(connectionString)

        If Not con.State = ConnectionState.Open Then

            con.Open()

        End If

        cmd.Connection = con

        Try

            cmd.CommandText = "INSERT INTO TR_RPRODUCT(ID_RPRODUCT, ID_PRODUCT, DATE_IN, STOCK_IN) VALUES ('" & Me.txtID_RPRODUCT.Text & "', '" & Me.cmbID_PRODUCT.SelectedValue & "', '" & _

                                Me.DateTimePicker1.Value.ToString("yyyy-MM-dd") & "', '" & Me.txtSTOCK_IN.Text & "')"

            cmd.ExecuteReader()

        Catch

            MsgBox("ERROR", MsgBoxStyle.OkOnly, "ERROR")

        End Try

        con.Close()

        If Not con.State = ConnectionState.Open Then

            con.Open()

        End If

        cmd2.Connection = con

        Try
                cmd2.CommandText = "SELECT * FROM MS_PRODUCT WHERE ID_PRODUCT='" & dgvTB.CurrentRow.Cells(1).Value & "'"

            stock_in = Convert.ToInt32(dgvTB.CurrentRow.Cells(3).Value)

            current_stock = Convert.ToInt32(cmd.ExecuteReader(5).ToString)

            qty = stock_in + current_stock

            con.Close()

            If Not con.State = ConnectionState.Open Then

                con.Open()

            End If

            cmd3.Connection = con

cmd3.CommandText = "UPDATE MS_PRODUCT SET CURRENT_STOCK='" & qty & "'WHERE ID_PRODUCT='" & dgvTB.CurrentRow.Cells(2).Value & "'"

            cmd3.ExecuteNonQuery()

            MsgBox("Stock increased", MsgBoxStyle.Information)

            con.Close()

        Catch

            MsgBox("Wrong Data", MsgBoxStyle.OkOnly, "Error")

        End Try

        con.Close()

        Me.GetData()

        Clear()

=====================


on the first command text looks fine (the data completely added to datagrid)

the issue is on the 2nd and 3rd command text (cmd2 and cmd3)
it wont read the script (i also use cmd2.ExecuteReader, but it contains error)

Violation of PRIMARY KEY constraint 'PK_TR_RPRODUCT'. Cannot insert duplicate key in object 'dbo.TR_RPRODUCT'. The statement has been terminated.

the STOCKS_IN from RPRODUCT should update the CURRENT_STOCKS from PRODUCT table.