PL/SQL :: Distinct Preceding Value Required

Following on from this answered thread: (xmlagg(xmlelement) - Distinct Values Required..Whereby the last poster recommended the use of

RTRIM (
XMLAGG (
XMLELEMENT (
E,
XMLATTRIBUTES (segment1|| ',' AS "Seg")
)
ORDER BY segment1 ASC
).EXTRACT ('./E[not(@Seg = preceding-sibling::E/@Seg)]/@Seg'),
','
)

To get a distinct list of elements. Works great, but I actually need to use the concept of "distinct immediately preceding".

Sample data:

create table xml_test
(emp_number number, seq number)

insert into xml_test values ('12345',1);
insert into xml_test values ('23456',2);
insert into xml_test values ('44323',3);
insert into xml_test values ('12345',4);
[code]....