[GENERAL] Block duplications in a shared buffers

Hi All,
I was analysing shared buffers content and noticed that exactly the same
disk block appears there many times with different or the same usagecount.
What's the cause of that? It's because of transactions? 

SELECT count(*), relfilenode, relblocknumber, array_agg(usagecount) usagecount, array_agg(isdirty)    isdirty,
array_agg(bufferid)  bufferid
 
FROM fo_main.buffercache
WHERE added = 'some_date'
GROUP BY relfilenode, relblocknumber
HAVING count(*) > 1
ORDER BY 1 DESC;

Sample result:

<http://www.postgresql-archive.org/file/t342733/Auswahl_830.png> 




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