Word Document content getting replaced

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C#

.NET

interop

In my application, I am trying to access word document programmatically.
My C# code will write a paragraph text (its a marker/separator) into the active word document at particular time interval.

That part is working perfectly.
The problem is, the same document is being accessed by another Application at the same time which write some text into it.

But when i add the marker at a particular interval, it replaces the existing text added by the other application.
But it retains older markers, only the content written by other application is getting replaced.

My code is copied, please help to figure out.

using System;
using System.Reflection;
using log4net;
using log4net.Config;
using Word = Microsoft.Office.Interop.Word;

namespace WordOperation{

class Program {

 private static ILog _log = LogManager.GetLogger(MethodBase.GetCurrentMethod().DeclaringType);
 private static object _oMissing = System.Reflection.Missing.Value;
 private static object _oEndOfDoc = "\\endofdoc";
 private static object _oEoF = Word.WdUnits.wdStory;

 static void Main(string[] args)
 {
   var wordApplication = new Word.ApplicationClass {Visible = true};
   Word.Document document = wordApplication.Documents.Add();
   document.Activate();
    while (true)
      {
      sleep(5000);
      object oRng = document.Bookmarks.get_Item(ref _oEndOfDoc).Range;
      Word.Paragraph markerParagraph = document.Content.Paragraphs.Add(ref oRng);
      markerParagraph.Range.Text = Common.MarkerTextContent;
      markerParagraph.Range.InsertParagraphAfter();
      wordApplication.ActiveWindow.Selection.EndKey(ref _oEoF, ref _oMissing);
      }
    }
  }
}