Hi Friend..
on insertion button click event just do this...
con.Open();
MySqlCommand cmd = new MySqlCommand("select * from Your_tableUser where name='"+txtboxName.text+"'", con);
MySqlDataReader dr = cmd.ExecuteReader();
if (dr.Read())
{
Lblmsg.text="Duplicate Entry Found...";
}
else
{
Here Do You Insert Code....
}
con.Close();
I think it will be helpful for you...