Using Grid view how to check Vendor name already exists

Question

i am doing application using grid view.in that i am inserting the record as VendorFName, VendorLName,VendorCity,VendorState,VendorCountry,VendorDescription etc.

in that i am using VendorID as Primary Key.

suppose when i insert one record VendorFName, VendorLName,VendorCity,VendorState,VendorCountry,VendorDescription it is saved in the database.

suppose again i am repeating the same VendorFName and click the new ADDROW Button i want to show the message VendorFName already exists.

for that how to write the code using csharp.

i send my code as follows;from my code how to write the code VendorFName already exists.

protected void ResultGridView_RowCommand(object sender, GridViewCommandEventArgs e)

    {

       if (e.CommandName.Equals("AddNew"))

        {

      

                TextBox txtFName = (TextBox)ResultGridView.FooterRow.FindControl("txtFName1");

                TextBox txtLName = (TextBox)ResultGridView.FooterRow.FindControl("txtLName1");

                TextBox txtCity = (TextBox)ResultGridView.FooterRow.FindControl("txtCity1");

                TextBox txtState = (TextBox)ResultGridView.FooterRow.FindControl("txtState1");

                TextBox txtCountry = (TextBox)ResultGridView.FooterRow.FindControl("txtCountry1");

                TextBox txtDescription = (TextBox)ResultGridView.FooterRow.FindControl("txtDescription1");

                SqlConnection conn = new SqlConnection("Server=(local);initial catalog=master;Trusted_Connection=True");

       

                cmd.Connection = conn;

                cmd.CommandText = "INSERT INTO Vendors(VendorFName, VendorLName,VendorCity,VendorState,VendorCountry,VendorDescription) Values('" + txtFName.Text + "', '" + txtLName.Text + "', '" + txtCity.Text + "', '" + txtState.Text + "', '" + txtCountry.Text + "' , '" + txtDescription.Text + "')";

                conn.Open();

                cmd.ExecuteNonQuery();

                FillVendorGrid();

                conn.Close();

            }

       }