How to close the form if more than one is open?

Question

private void dgProductsView_CellClick(object sender, DataGridViewCellEventArgs e)
{
    AddProduct addproduct = new AddProduct();
    addproduct.txtName.Text = dgProductsView.SelectedRows[0].Cells[1].Value + string.Empty;
    addproduct.txtCategory.Text = dgProductsView.SelectedRows[0].Cells[2].Value + string.Empty;
    addproduct.txtSupplier.Text = dgProductsView.SelectedRows[0].Cells[3].Value + string.Empty;
    addproduct.txtMFG.Text = dgProductsView.SelectedRows[0].Cells[4].Value + string.Empty;
    addproduct.txtEXP.Text = dgProductsView.SelectedRows[0].Cells[5].Value + string.Empty;
    addproduct.txtUnit.Text = dgProductsView.SelectedRows[0].Cells[6].Value + string.Empty;
    addproduct.txtSelling.Text = dgProductsView.SelectedRows[0].Cells[7].Value + string.Empty;
    addproduct.txtOriginal.Text = dgProductsView.SelectedRows[0].Cells[8].Value + string.Empty;
    addproduct.txtQuantity.Text = dgProductsView.SelectedRows[0].Cells[9].Value + string.Empty;
    addproduct.txtStock.Text = dgProductsView.SelectedRows[0].Cells[10].Value + string.Empty;
    addproduct.txtTotal.Text = dgProductsView.SelectedRows[0].Cells[11].Value + string.Empty;

    if(addproduct is open)
    {
        addproduct.Close();
    }
    else
    {
        addproduct.Show();
    }