This seems so unlikely I'm sure something has gone wrong somewhere, but I can't begin to figure out how or where. Using nothing more than the standard Sitefinity backend admin pages to manually populate an image library and a dynamic module content type (the former many months ago, the latter in just the last couple of months), I've now got two completely unrelated items with the same GUID: 6F5C9D52-002B-6B40-82E1-FF0000EEE7A0.
1) The first instance of this GUID is in the base_id of a published content item in the custom dynamic module table bvrarticles_article.
2) This same GUID is also the id of an entry in the sf_media_file_links table (and the media_file_link_id of a related entry in the sf_media_file_urls table).
These two things have nothing at all to do with one another.
How is it possible that duplicate GUIDs would have been generated like this? Is this something you all have seen before in Sitefinity? (Note we started out in v8 and converted up to v10 earlier this year.)
In this case, the duplicate GUID shouldn't be an issue because these unrelated things are accessed with entirely different APIs, and I can simply unpublish the dynamic module content item and republish the master to generate a new base_id. But this really shouldn't happen, so it suggests that something has gone wrong somewhere.
Well, I'd guess this is the problem:
We've got the incremental scheme turned on but have not setup different site synchronization keys on production, test, and local development.