title: EPPLUS is duplicating strings on insert
body: <p>I am trying to insert text into RichText, when the index of the inserted string is at the end of an element the next element gets duplicated!</p>
  
  <p>Here is an example:</p>
  
  <pre><code>worksheet.Cells[rownum + 100, column].RichText.Add("first ");
  worksheet.Cells[rownum + 100, column].RichText.Add(" second");
  worksheet.Cells[rownum + 100, column].RichText.Text = worksheet.Cells[rownum + 100, column].RichText.Text.Insert(6, "Inserted");
  </code></pre>
  
  <p>Result: "first Insertedsecondsecond"</p>
  
  <p>Is this normal behavior? because I am expecting to get:</p>
  
  <p>"first Inserted second"</p>