How to solve this error

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ASP.NET

I am getting this error..
"Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression."
I have written this procedure..
///////////////////////////////////////////////////////////////////////////////

alter procedure myprocedure
(
@flag varchar(1),
@id  varchar(25),
@amt decimal(10,2)
)
as
begin
	if @flag='D'
	begin
		declare @balance decimal(10,2)
			set @balance=(select balance from mytransaction where emailid=@id);
			if (@balance IS NULL)
			begin
				set @balance=@amt;
			end
			else
			begin
			set @balance=@balance+@amt;
			end
			insert into mytransaction values(@id,@balance,GETDATE(),'DEPOSIT',@amt)
	end
	    else if @flag='W'
	       begin
			declare @chkbalance decimal(10,2)
			set @chkbalance=(select balance from mytransaction where emailid=@id);
				if (@amt>@chkbalance)
				begin
					SELECT 'Amount cannot be greater than balance.' AS 'Result'
				end
				  else 
					begin
					declare @wbalance decimal(10,2)
			set @wbalance=(select balance from mytransaction where emailid=@id);
			set @wbalance=@wbalance-@amt;
			insert into mytransaction values(@id,@wbalance,GETDATE(),'WITHDRAW',@amt)
			SELECT 'You have withdraw ammount of -->' + str(@amt) AS 'Result'
			end
	        end
end

//////////////////////////////////////////////////////////////////////////////

and have written follwing line of code in code file
///////////////////////////////////////////////////////////////////////////////

protected void btntransaction_Click(object sender, EventArgs e)
    {
        cn.Open();
        if (rdbwithdraw.Checked == true)
        {
            cmd = new SqlCommand("myprocedure", cn);
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.AddWithValue("@flag", SqlDbType.VarChar).Value = "W";
            cmd.Parameters.AddWithValue("@id", SqlDbType.VarChar).Value = hdnname.Value;
            cmd.Parameters.AddWithValue("@amt",SqlDbType.Decimal).Value = txtamt.Text;           
            da = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            da.Fill(dt);

            if (dt != null && dt.Rows.Count > 0)
                Response.Write(dt.Rows[0]["Result"].ToString());

        }
        else
        {
            cmd = new SqlCommand("myprocedure", cn);
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.AddWithValue("@flag", SqlDbType.VarChar).Value = "D";
            cmd.Parameters.AddWithValue("@id", SqlDbType.VarChar).Value = hdnname.Value;
            cmd.Parameters.AddWithValue("@amt", SqlDbType.Decimal).Value = txtamt.Text;
            cmd.ExecuteNonQuery();
            Response.Write("You have deposited ammount of -->" + txtamt.Text);
            txtamt.Text = "";

        }
        cn.Close();
    }