--- a/Mailman/MailList.py	
+++ a/Mailman/MailList.py	
@@ -589,7 +589,7 @@ class MailList(HTMLFormatter, Deliverer, ListAdmin,
             # to ENOENT (EnvironmentError is the base class of IOError and
             # OSError).
             mtime = os.path.getmtime(dbfile)
-            if mtime <= self.__timestamp:
+            if mtime < self.__timestamp:
                 # File is not newer
                 return None, None
             fp = open(dbfile)
--- a/Mailman/Queue/IncomingRunner.py	
+++ a/Mailman/Queue/IncomingRunner.py	
@@ -130,6 +130,8 @@ class IncomingRunner(Runner):
             more = self._dopipeline(mlist, msg, msgdata, pipeline)
             if not more:
                 del msgdata['pipeline']
+            while not mlist.Locked():
+                mlist.Lock(timeout=mm_cfg.LIST_LOCK_TIMEOUT)
             mlist.Save()
             return more
         finally: