This patch enables to avoid sending duplicate mail if
other list address is specified in to: or cc: header.
- Sibling lists are other mailing lists on this site
whose members are excluded from regular delivery if
those list addresses appear in To: or Cc: header.
- The list addresses should be witten in full mail
address format (e.g. email@example.com). Do not
specify the list address mutually in the sibling list
configuration page or those doubled members won't get
- Is the terminology 'sibling' appropriate?
- Need more comments in the processing code