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diff --git a/src/mailman/handlers/avoid_duplicates.py b/src/mailman/handlers/avoid_duplicates.py
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+++ b/src/mailman/handlers/avoid_duplicates.py
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+# Copyright (C) 2002-2012 by the Free Software Foundation, Inc.
+#
+# This file is part of GNU Mailman.
+#
+# GNU Mailman is free software: you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or (at your option)
+# any later version.
+#
+# GNU Mailman is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+# You should have received a copy of the GNU General Public License along with
+# GNU Mailman.  If not, see <http://www.gnu.org/licenses/>.
+
+"""If the user wishes it, do not send duplicates of the same message.
+
+This module keeps an in-memory dictionary of Message-ID: and recipient pairs.
+If a message with an identical Message-ID: is about to be sent to someone who
+has already received a copy, we either drop the message, add a duplicate
+warning header, or pass it through, depending on the user's preferences.
+"""
+
+from __future__ import absolute_import, unicode_literals
+
+__metaclass__ = type
+__all__ = [
+    'AvoidDuplicates',
+    ]
+
+
+from email.utils import getaddresses, formataddr
+from zope.interface import implements
+
+from mailman.core.i18n import _
+from mailman.interfaces.handler import IHandler
+
+
+COMMASPACE = ', '
+
+
+
+class AvoidDuplicates:
+    """If the user wishes it, do not send duplicates of the same message."""
+
+    implements(IHandler)
+
+    name = 'avoid-duplicates'
+    description = _('Suppress some duplicates of the same message.')
+
+    def process(self, mlist, msg, msgdata):
+        """See `IHandler`."""
+        recips = msgdata.get('recipients')
+        # Short circuit
+        if not recips:
+            return
+        # Seed this set with addresses we don't care about dup avoiding.
+        listaddrs = set((mlist.posting_address,
+                         mlist.bounces_address,
+                         mlist.owner_address,
+                         mlist.request_address))
+        explicit_recips = listaddrs.copy()
+        # Figure out the set of explicit recipients.
+        cc_addresses = {}
+        for header in ('to', 'cc', 'resent-to', 'resent-cc'):
+            addrs = getaddresses(msg.get_all(header, []))
+            header_addresses = dict((addr, formataddr((name, addr)))
+                                    for name, addr in addrs
+                                    if addr)
+            if header == 'cc':
+                # Yes, it's possible that an address is mentioned in multiple
+                # CC headers using different names.  In that case, the last
+                # real name will win, but that doesn't seem like such a big
+                # deal.  Besides, how else would you chose?
+                cc_addresses.update(header_addresses)
+            # Ignore the list addresses for purposes of dup avoidance.
+            explicit_recips |= set(header_addresses)
+        # Now strip out the list addresses.
+        explicit_recips -= listaddrs
+        if not explicit_recips:
+            # No one was explicitly addressed, so we can't do any dup
+            # collapsing
+            return
+        newrecips = set()
+        for r in recips:
+            # If this recipient is explicitly addressed...
+            if r in explicit_recips:
+                send_duplicate = True
+                # If the member wants to receive duplicates, or if the
+                # recipient is not a member at all, they will get a copy.
+                # header.
+                member = mlist.members.get_member(r)
+                if member and not member.receive_list_copy:
+                    send_duplicate = False
+                # We'll send a duplicate unless the user doesn't wish it.  If
+                # personalization is enabled, the add-dupe-header flag will
+                # add a X-Mailman-Duplicate: yes header for this user's
+                # message.
+                if send_duplicate:
+                    msgdata.setdefault('add-dup-header', set()).add(r)
+                    newrecips.add(r)
+                elif r in cc_addresses:
+                    del cc_addresses[r]
+            else:
+                # Otherwise, this is the first time they've been in the recips
+                # list.  Add them to the newrecips list and flag them as
+                # having received this message.
+                newrecips.add(r)
+        # Set the new list of recipients.  XXX recips should always be a set.
+        msgdata['recipients'] = list(newrecips)
+        # RFC 2822 specifies zero or one CC header
+        if cc_addresses:
+            del msg['cc']
+            msg['CC'] = COMMASPACE.join(cc_addresses.values())