SELECT bug?

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POSTGRESQL BUG REPORT TEMPLATE
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Your name : Jacek Kalinski (Jack)
Your email address : jacek_kal(at)o2(dot)pl

System Configuration
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Architecture (example: Intel Pentium) : i686 - AMD Athlon XP 1800+

Operating System (example: Linux 2.0.26 ELF) : Linux2.4.20/2.4.21 ELF

PostgreSQL version (example: PostgreSQL-7.3.3): PostgreSQL-7.3.3

Compiler used (example: gcc 2.95.2) : 3.2.2

Memory: 512MB DDR
Avg CPU usage when inserting records to db: 20-35%

Please enter a FULL description of your problem:
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Firstly, sorry for my English (it is not my native language).

I have a strange problem: I created a new database in postgres. I tried
to import a tables, views etc. - there was no error.
Then I started a small script (wroted in perl) which started to import
data into my database - but I think that is no problem with this script..
After that I checked, that records are incorrect (are duplicated!).
It is not first time, when records in different tables are duplicated.
In another console i tried to connect to this database (by psql). I wrote:
select usable,count(*) from authors group by usable having count(*)>1;

I get a list of duplicates. When I tried to select only duplicated rows
by select * from authors where usable = '"name" <alias(at)domain(dot)pl>';
first run this command returned me 0 rows (sometimes, but rarely I also
got that 1 rows were found as you can see below):
id | original | usable
------+--------------------------+--------------------------------
13126 | "name" <alias(at)domain(dot)pl> | "name" <alias(at)domain(dot)pl>
(1 row)

but when I wrote the same command not with equals '=' but with 'like' I
got:
id | original | usable
------+--------------------------+--------------------------------
8078 | "name" <alias(at)domain(dot)pl> | "name" <alias(at)domain(dot)pl>
13126 | "name" <alias(at)domain(dot)pl> | "name" <alias(at)domain(dot)pl>
(2 rows)

I really do not know where can be a problem.
Now it is not a problem that records were inserted twice (or more - I
also have this records), but the script is checking if record already
exist by select.

I don't know what to do with this...

Please describe a way to repeat the problem. Please try to provide a
concise reproducible example, if at all possible:
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How to repeat this problem? It is very easy: just connect to database
and write down select command.
To get different records it is very difficult, but why select does not
return correctly (0 rows)?

Yours faithfully,
Jack Kalinski