Try this (warning not tested)
cmd.Parameters.Add("@sm", SqlDbType.NVarChar);
cmd.Parameters.Add("@st", SqlDbType.NVarChar);
cmd.Parameters.Add("@est", SqlDbType.NVarChar);
while ((line = sr.ReadLine()) != null)
{
string[] L = Regex.Split(line, ",");
conn.Open();
if (line != lastline)
{
cmd.Parameters["@sm"].Value = line[0];
cmd.Parameters["@st"].Value = line[1];
cmd.Parameters["@est"].Value = line[3];
cmd.ExecuteNonQuery();
lastline = line;
}
conn.Close();
}