Did you executed the manual upgrade or automatic via KIM?
It seems that there is an inconsistence in the DB. Could you please check the CMS_UIElement table and search for any records that have the same ElementResourceID - duplicates? If possible, I would recommend to remove the duplicates form the DB and then try to repeat the operation.
Also, were there added any custom elements in version 7? If so, please make sure they are marked as custom before upgrading. Otherwise the upgrade thinks they are the default ones and tries to upgrade them as well.
The DB should be OK though. There could be some obsolete UI elements and resources that were removed in Kentico 8.
You can run these two queries to get the old UI elements and then you can delete those elements - either through UI or in the DB directly (please, be sure you have actual backup of the DB ready - just in case).
Where ElementResourceID IN
WHERE ResourceGUID = '07a7176d-b966-43dc-ae21-665599b92f7d'
WHERE ElementIDPath LIKE '%/' + CONVERT (nvarchar, (Select ElementID FROM CMS_UIElement WHERE ElementGUID = 'fb9af80e-8d78-481e-97cb-b276a673b8ba')) + '/%'