From user@domain  Fri Feb 22 15:06:23 2008
Message-Id: <a@b>

body1

From user@domain  Fri Feb 22 15:06:23 2008
Message-Id: <b@b>
References: <a@b>

body2

From user@domain  Fri Feb 22 15:06:23 2008
Message-Id: <c@b>
References: <b@b>

body3

From user@domain  Fri Feb 22 15:06:23 2008
Message-Id: <b@b>
References: <a@b>

body4 (2 duplicate)