Outlook has already begun transmitting this message

[Dorian]

One of my users has been getting this message.

System.Runtime.InteropServices.COMException (0xBBC40114): Outlook has
already begun transmitting this message.
at Microsoft.Office.Interop.Outlook._MailItem.get_EntryID()
at PeiceOutlookSupportTracker.ThisAddIn.Explorer_SelectionChange()

Code:

private void Explorer_SelectionChange()
{
try
{
if (m_Explorer != null)
{
// We only care if there is a single collection
Selection sel = m_Explorer.Selection;
if (sel.Count == 1)
{
object item = sel[1];
if (item is MailItem)
{
MailItem mail = (MailItem)item;

if (vmHandler.LastItemId != mail.EntryID)
{
...Additional code here...
}
}
}
}
}
}
catch (Exception ex)
{
// Log error here
}

Anyone know what exactly is going on and how to make sure it doesn't occur?