Obj[0]: stock_symbol, stock_price...

Obj[1]: stock_symbol, stock_price...

Obj[2]: stock_symbol, stock_price...

Obj[3]: stock_symbol, stock_price...

for(int i = 0;  i < j; i++)
{
    DataRow itemRow = comboBoxItems.NewRow();
    Stock_ComboBox.Items.Add(dTOs[i].stock_symbol);
}

if (Stock_ComboBox.Text == dTOs[i].stock_symbol)
{
    continue;
} else
{
    DataRow itemRow = comboBoxItems.NewRow();
    Stock_ComboBox.Items.Add(dTOs[i].stock_symbol);
}

for( .....
{
Stock_ComboBox.Items.Add(dTOs[i].stock_symbol.Distinct().ToString());
}