fix 240465 new flag on local folder which has messages filtered into it doesn't get cleared when last new message is read, sr=mscott

mailnews/db/msgdb/src/nsMsgDatabase.cpp

@@ -2252,7 +2252,9 @@ NS_IMETHODIMP nsMsgDatabase::MarkReadByDate (PRTime startDate, PRTime endDate, n
 
 NS_IMETHODIMP nsMsgDatabase::AddToNewList(nsMsgKey key)
 {
-  return m_newSet.Add(key);
+  if (m_newSet.IndexOf(key) == kNotFound)
+    m_newSet.Add(key);
+  return NS_OK;
 }