Outlook VSTO Add-In Sending Meeting requests Twice

[BigDubb]

We have created a VSTO addin for Outlook Meetings.

As part of this we trap on the SendEvent of the message on the
FormRegionShowing event:
_apptEvents.Send += new
Microsoft.Office.Interop.Outlook.ItemEvents_SendEventHandler(_apptEvents_Send);

The method _apptEvents_Send then tests on a couple of properties

private void _apptEvents_Send(ref bool Cancel)
{
if (!_Qualified)
{
MessageBox.Show("Meeting has not been qualified",
"Not Qualified Meeting", MessageBoxButtons.OK, MessageBoxIcon.Information);
chkQualified.Focus();
Cancel = true;
}
}

The problem that we're having is that soem users' messges get sent twice.
Once when the meeting is sent and a second time when the user re-opens
outlook.

Does anyone have any ideas on what might be causing this?