unique index creation failure in 7.4.1 - bug?

Did a pg_dump of a 7.2 DB and am now trying to restore
to 7.4.1. Index creation on a table fails:

db=# CREATE UNIQUE INDEX person_info_username_ix ON
person_info USING btree (username);
ERROR: could not create unique index
DETAIL: Table contains duplicated values.

Of course, this index existed in the 7.2 DB, so it
seems odd that it should not be possible to recreate.
Furthermore, a search for duplicate values fails:

db=# select username, count(*) from person_info group
by username having count(*) > 1;
username | count
----------+-------
(0 rows)

Is this a known issue?

Ben

__________________________________
Do you Yahoo!?
Yahoo! Hotjobs: Enter the "Signing Bonus" Sweepstakes
http://hotjobs.sweepstakes.yahoo.com/signingbonus