Symptoms

1. Cannot rebuild indicies from scratch after upgrade
2. Cannot search successfully

The following appears in the atlassian-confluence.log:

org.springframework.dao.IncorrectResultSizeDataAccessException: Multiple AttachmentData objects were returned when only one was expected for attachment: Attachment: <attachment name and id>
        at com.atlassian.confluence.pages.persistence.dao.hibernate.HibernateAttachmentDataDao.getAttachmentDataForAttachment(HibernateAttachmentDataDao.java:77)

Cause

There are duplicates in the attachmentdata table, most likely due to this bug  CONF-7882 - Getting issue details... STATUS

Resolution

1. Identify duplicates from attachmentdata table in the database

   select attachmentdataid, attachmentid, attversion, count(*) from ATTACHMENTDATA group by attachmentid, attachmentdataid, attversion having count(*)>1;

2. Delete the duplicates that have the same attachmentid and attversion, leaving the version which has the greatest attachmentdataid:

   delete from ATTACHMENTDATA where attachmentdataid=<put in attachmentdataids here>;

3. Last step is to How to Rebuild the Content Indexes From Scratch on Confluence Server