In multiwiki mode, class documents are randomly overwritten by empty docs in the cache

I noticed this on a farm where copyDocument was randomly skipping class documents when creating a new wiki from a template wiki.
The reason copyDocument was skipping is because sourceDoc.isNew() is returning 'true', even if the document is present in the database.

When creating multiple wikis in a row:

• the first wiki is always correctly created: it contains all the documents from the template
• the following wikis are missing a couple of classes, but never the same classes