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diff --git a/Mailman/Handlers/AvoidDuplicates.py b/Mailman/Handlers/AvoidDuplicates.py
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+++ b/Mailman/Handlers/AvoidDuplicates.py
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+# Copyright (C) 2002 by the Free Software Foundation, Inc.
+#
+# This program 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 2
+# of the License, or (at your option) any later version.
+# 
+# This program 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 this program; if not, write to the Free Software 
+# Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+
+"""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 Mailman import mm_cfg
+
+from email.Utils import getaddresses, formataddr
+
+
+
+def process(mlist, msg, msgdata):
+    recips = msgdata['recips']
+    # Short circuit
+    if not recips:
+        return
+    # Seed this set with addresses we don't care about dup avoiding
+    explicit_recips = {}
+    listaddrs = [mlist.GetListEmail(), mlist.GetBouncesEmail(),
+                 mlist.GetOwnerEmail(), mlist.GetRequestEmail()]
+    for addr in listaddrs:
+        explicit_recips[addr] = 1
+    # Figure out the set of explicit recipients
+    ccaddrs = {}
+    for header in ('to', 'cc', 'resent-to', 'resent-cc'):
+        addrs = getaddresses(msg.get_all(header, []))
+        if header == 'cc':
+            for name, addr in addrs:
+                ccaddrs[addr] = name, addr
+        for name, addr in addrs:
+            if not addr:
+                continue
+            # Ignore the list addresses for purposes of dup avoidance
+            explicit_recips[addr] = 1
+    # Now strip out the list addresses
+    for addr in listaddrs:
+        del explicit_recips[addr]
+    if not explicit_recips:
+        # No one was explicitly addressed, so we can't do any dup collapsing
+        return
+    newrecips = []
+    for r in recips:
+        # If this recipient is explicitly addressed...
+        if explicit_recips.has_key(r):
+            send_duplicate = 1
+            # If the member wants to receive duplicates, or if the recipient
+            # is not a member at all, just flag the X-Mailman-Duplicate: yes
+            # header.
+            if mlist.isMember(r) and \
+                   mlist.getMemberOption(r, mm_cfg.DontReceiveDuplicates):
+                send_duplicate = 0
+            # 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', {})[r] = 1
+                newrecips.append(r)
+            elif ccaddrs.has_key(r):
+                del ccaddrs[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.append(r)
+    # Set the new list of recipients
+    msgdata['recips'] = newrecips
+    del msg['cc']
+    for item in ccaddrs.values():
+        msg['cc'] = formataddr(item)