XML Space Import fails with "different object with the same identifier value" Error

Question

I have a space which was exported with Confluence 6.10.1 server and fails when imported into Confluence 6.11.0 server. Other space imports work fine but this one however fails with the error message down below. I even created a fresh new DB and only imported this space and it still fails. How can I fix this error or identify what's causing it? com.atlassian.confluence.importexport.ImportExportException: Unable to complete import: Error while importing backup: A different object with the same identifier value was already associated with the session : [bucket.user.propertyset.BucketPropertySetItem#BucketPropertySetItem[entityId=327876,entityName=confluence_ContentEntityObject,key=metadata.{toc:outline=true} {info:title=Subpages for this project} {children} {info} {tip:title=Attachements for this project} {attachments} {tip},type=6,boolean=false,string=,text=,int=0,double=0.0,long=0,date=]] ... Caused by: org.xml.sax.SAXException: Error while importing backup: A different object with the same identifier value was already associated with the session : [bucket.user.propertyset.BucketPropertySetItem#BucketPropertySetItem[entityId=327876,entityName=confluence_ContentEntityObject,key=metadata.{toc:outline=true} ... Caused by: net.sf.hibernate.cache.CacheException: A different object with the same identifier value was already associated with the session : [bucket.user.propertyset.BucketPropertySetItem#BucketPropertySetItem[entityId=327876,entityName=confluence_ContentEntityObject,key=metadata.{toc:outline=true} {info:title=Subpages for this project} {children} {info} {tip:title=Attachements for this project} {attachments} {tip},type=6,boolean=false,string=,text=,int=0,double=0.0,long=0,date=]]

Thomas Buchhorn · Answer

Hi Diego,

I managed to create a space with only a few pages which triggers this issue.

I could send you this space as XML export file for investigation. All confidential information has been removed from the pages.

Thomas Buchhorn · Answer

I had two spaces with this issue and was able to solve my issue.

One space import I fixed by deleting the entire history of a certain page.

The other space with the error above I fixed by changeing this line

{info:title=Subpages for this project}

to

{info:title=Subpages for this projectxyz}

There have been 6 Entities with in the XML and changing one of them fixed the issue.

Maybe I'm able to create a sample import file to reproduce this error but I'm now away for two weeks.

Diego_88dj6o · Answer

Hello Thomas! Quite an intriguing situation. 
Looking a little further, the line:

Unable to complete import: Error while importing backup: A different object with the same identifier value was already associated with the session : [bucket.user.propertyset.BucketPropertySetItem#BucketPropertySetItem

Could give us a lead. You could have been affected by something similar to this:

CONFSERVER-36318

I was unable to reproduce this locally on Confluence 6.9 and 6.10. 
You could try the following:

1- Remove page restrictions from pages
2- Export the space again using a full space export (do not select the custom export)
3- Try importing it to the same Confluence version
4- If the same version works, try importing into 6.11 again

Let us know if you are able to follow through with this and if it helped your case!