The variable name '@' has already been declared. Variable names must be unique within a query...

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

When I execute this code below, above error comes on my sql parameters on second @LockState.

private void btn_lock2_Click(object sender, EventArgs e)
{
rwd.command = new SqlCommand();
rwd.command.Connection = rwd.connection;

try
{
if ((txt2.Text == "")| (txt_desc2.Text == ""))
appMessages.unCompleteFields();
else
{
long from = long.Parse(this.txt2.Text);
long to = long.Parse(this.txt3.Text);
if (from <= to)
{
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.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();
}
appMessages.successfulyUpdated();
}
else
{
appMessages.unsuccessfulyUpdated();
}
this.txt1.Text = "";
this.txt2.Text = "";
}
}
catch (Exception exp) { throw exp; }
}
}

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