The variable name ‘@’ has already been declared. Variable names must be unique within a query batch or stored procedure

by

You are adding multiple times the same parameters in every iteration of loop.

Add rwd.command.Parameters.Clear() after each loop iteration: 

for (long counter = from; counter <= to; counter++)
{
    rwd.command.Parameters.Clear();

    string upd = "update card set LockState=@lockstate, card_descr=@card_descr where [cardNumber] = N'{0}'";
    rwd.command.CommandText = upd;
    rwd.command.Parameters.Add(new SqlParameter("@LockState",
    SqlDbType.NVarChar)).Value =1;
    rwd.command.Parameters.Add(new SqlParameter("@card_descr",
    SqlDbType.NVarChar)).Value = txt_desc2.Text;
    rwd.connection.Open();
    rwd.command.ExecuteScalar();
    rwd.connection.Close();
}

or add parameter before loop:

rwd.command.Parameters.Add(new SqlParameter("@LockState", SqlDbType.NVarChar));
rwd.command.Parameters.Add(new SqlParameter("@card_descr", SqlDbType.NVarChar));

and then in loop:

for (long counter = from; counter <= to; counter++)
{
    string upd = "update card set LockState=@lockstate,
    card_descr=@card_descr where [cardNumber] = N'{0}'";
    rwd.command.CommandText = upd;

    rwd.command.Parameters["@LockState"].Value =1;
    rwd.command.Parameters["@card_descr"].Value = txt_desc2.Text;

    rwd.connection.Open();
    rwd.command.ExecuteScalar();
    rwd.connection.Close();
}

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