Preventing repeated data into the gridview

Question

I have a .net program in which datas are to be inserted through gridview. If I am adding the same data twice then I need to show error message that data repeated. How can I do this. I have used one code but it shows error.

 

for (int i = 0; i < GridView1.Rows.Count; i++)

        {

            TextBox txtSlno = (TextBox)GridView1.Rows[i].Cells[1].FindControl("txtSlno");

            if (txtSlno.Text != "" || txtSlno.Text != null)

            {

                if (txtSlno.Text == dt.Rows[i]["SlNo"].ToString())

                {

                    Label1.Text = "already exist";

                    break;

                }

                else

                {

                    Label1.Text = "";

                }

            }

        }

It shows error there is no row at position zero.