How to update data when it is appears in more than one section by LINQ

see the screen shot 

see LineItem R&D and XfundCode RD appears in consensus and also in key drug section. i want if any specific combination of LineItem & XfundCode appears in multiple section then xfundcode will be empty from the second occurrence.

xfundcode is attached with line item. it is a property of line item.

so output will be....screen shot attached 

i tried this code but did not get desire output.

namespace LINQ_Remove_Duplicate
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void button1_Click(object sender, EventArgs e)
        {
            List<Data> _lstMainData = new List<Data>
            {
                new Data() { Section="Consensus", Lineitem = "R&D", xfundcode = "RD" },
                new Data() { Section="Consensus",Lineitem = "SG&A", xfundcode = "SGA" },
                new Data() { Section="Key Drugs",Lineitem = "R&D", xfundcode = "RD" },
                new Data() { Section="Key Drugs",Lineitem = "SG&A", xfundcode = "SGA" }
            };

            var groupResults = _lstMainData
                .GroupBy(data => new
                {
                    LineItem = data.Lineitem,
                    xFundcode = data.xfundcode
                })
                .Select(g => new
                {
                    Count = g.Count(),
                    LineItem = g.Key.LineItem,
                    xfundcode = g.Key.xFundcode,
                    List = g.ToList()

                })
                .Where(dataGroup => dataGroup.Count > 1)
                .ToList();

            for (int index = 0; index < groupResults.Count; index++)
            {
                for (int listIndex = 1; listIndex < groupResults[index].List.Count; listIndex++)
                {
                    groupResults[index].List[listIndex].xfundcode = "";
                }
            }
        }
    }

    public class Data
    {
        public string Section { get; set; }
        public string Lineitem { get; set; }
        public string xfundcode { get; set; }
    }
}

please share the right code sample which will find & update xfundcode to empty value if any combination of line item and xfund code appear in more than one section.

thanks