ProcessedFile/Thumbnail is always regenerated
A ProcessedFile/Thumbnail is always regenerated even if there are not modifications to the original file.
The reason for this spread all over the ProcessedFile handling and boils down to a new ProcessedFile is being persisted in the file-system by using a regular ResourceStorage::addFile() call that results in a plain File object. The indexer then tries to load this file and will create a new UID for the thumbnail in sys_file. This uid value is then used to update(!) the sys_file_processedfile entry - which fails since the uid was taken from sys_file.
Since this is spead a lot, the only chance to overcome this, is to halt the indexer if the file identifier starts with the processing folder identifier.