SQL Insert and Update Commands in vb.net

Hi Experts,

I have some trouble with sql commands in VB.net,  

On a winform, I am trying to insert new data to Ms sql database table, each data has its own text.box areas.

my codes,

    Dim adptr2 As New SqlDataAdapter("select * from  STOKANA", baglan)
    Dim stkekle As New SqlCommand("insert into STOKANA values(@SN,@SA,@SM,@SB,@EM,@RD)", baglan)
    Dim ds1 As New DataSet


button click event
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        If TextBox1.Text = "" Then
            MessageBox.Show("Stok adi giriniz.", "STOK ADI", MessageBoxButtons.OK, MessageBoxIcon.Warning)
            TextBox1.Focus()
        ElseIf TextBox2.Text = "" Then
            MessageBox.Show("Stok miktarini giriniz.", "STOK MIKTARI", MessageBoxButtons.OK, MessageBoxIcon.Warning)
            TextBox2.Focus()
        ElseIf ComboBox1.Text = "" Then
            MessageBox.Show("Stok birimini giriniz.", "STOK BIRIMI", MessageBoxButtons.OK, MessageBoxIcon.Warning)
            TextBox2.Focus()
        ElseIf TextBox4.Text = "" Then
            MessageBox.Show("Stok miktarini giriniz.", "STOK MIKTARI", MessageBoxButtons.OK, MessageBoxIcon.Warning)
            TextBox4.Focus()
        Else

            Try
                stkekle.Parameters.AddWithValue("SN", TextBox10.Text)
                stkekle.Parameters.AddWithValue("SA", TextBox1.Text)
                stkekle.Parameters.AddWithValue("SM", TextBox2.Text)
                stkekle.Parameters.AddWithValue("SB", ComboBox1.Text)
                stkekle.Parameters.AddWithValue("EM", TextBox4.Text)
                stkekle.Parameters.AddWithValue("RD", CheckBox1.CheckState)
                baglan.Open()
                adptr2.InsertCommand = stkekle
                stkekle.ExecuteNonQuery()
                MsgBox("Kayit eklendi !!")
            Catch ex As Exception
                MsgBox(ex.ToString)
            End Try
            baglan.Close()
            Call veri1()
            End If
    End Sub

after adding first data row on sql table, I could not add second data row  and the error massage is that "sql data sqlcient sql expection (0x80131904). The variable name '@SN' has already been declared.  


The update command

I am using datagrid to show user which data he wanna update and data is going to on  textboxes and checkboxes on the form by clicking on selected datagird row object.
when updating the data,  the error massage is that sql data sqlcient sql expection (0x80131904). Error converting data type nvarchar to numeric.

Please help

Thank you

Oguzhan