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If you have duplicate URLs in your broker, your website will be unhappy at best and will probably throw an error when it tries to get the content of the page from the broker for a given URL.
This situation can occur when you publish a page for example called "index.htm" then you rename the page to "index_old.html" and then create a new page called "index.htm" and publish out that new page.
to find all the duplicates in the Broker DB run the following
select *
from PAGE a
join (SELECT PUBLICATION_ID , URL
FROM PAGE
GROUP BY PUBLICATION_ID, URL
having count (*) > 1 ) b
on a .PUBLICATION_ID = b.PUBLICATION_ID
and a .URL = b.URL
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Run as a read only user, not supported by SDL support etc.