I think, If you created the page with older version with duplicate attachments and still you can see this error in latest versions when you view.

Seen on 5.4.4

Resolved????

Seen on 4.2.13 with no duplicate attachments

Seen on 5.1.5

same error found multiple times in Confluence 4.3.7

e.g.
on attachments:
2013-04-25 13:25:14,627 ERROR [http-8180-39] [com.atlassian.hibernate.HibernateObjectDao] findSingleObject Uh oh - found more than one object when single object requested: (the following details lists the name of an attachment twice)

on notifications:
2013-04-26 10:23:43,968 ERROR [StreamsCompletionService::thread-1350] [com.atlassian.hibernate.HibernateObjectDao] findSingleObject Uh oh - found more than one object when single object requested: [com.atlassian.confluence.mail.notification.Notification@17581f1, com.atlassian.confluence.mail.notification.Notification@17581f2]

This issue duplicates CONF-7882

We have 3 of these records on the atlassian extranet. It is (at least in our case) the attachmentdata that has the duplicate rows. Two attachmentdata records pointing to the same row of the attachments table.

I've added the clustering component because both our instance and the support case that created this issue were clusters, which may have been the cause. Of course, clusters always have attachmentdata tables and most instances don't.

This state prevents migrating the attachments to the filesystem at the moment.

If you view the attachments for the page it will display two entries for the same attachment version, but clicking either of them results in a message that the data is missing. (And logging saying there are two of them).

If we add an appropriate unique constraint to the attachmentdata table then this becomes an impossible bug, but an upgrade task must also be added to clean up the data beforehand.

I was able to fix the error by going to a page that I could edit and changing the pageId at the end to the page that was broken. Once I was able to edit it, I just changed what the page contained on it and everything started working again.