BUG #6498: with recursive / union all

The following bug has been logged on the website:

Bug reference: 6498
Logged by: kanishka
Email address: nish2575(at)gmail(dot)com
PostgreSQL version: 9.0.6
Operating system: Linux 2.6.41.4-1.fc15.i686 / fedora 15
Description:

here is an example of what i believe the system is not obeying the
expectation of a union all instead of a union, in a recursive with query
that uses a join. the documentation page on with queries also has an example
with union all over an acyclic graph data set that i believe should loop
infinitely according the descriptoin of with query processing.

this doesn't loop infinitely:

insert into acyc values ('a','b') ,('b','c');

with recursive paths as (
select frm, too, 1 as lvl from acyc
union all
select fnd.frm, a.too, lvl + 1 from acyc a join paths fnd on fnd.too =
a.frm ) select * from paths;

while this does loop infinitely:

with recursive p as (select 5 union all select * from p ) select * from p;

my guess is the performing of the join is causing duplicates inadvertantly.