If a page is moved without a redirect while someone is editing it, rather than an edit conflict being generated, a duplicate page will be created. This is mostly a problem with new pages created by new users, which on the English Wikipedia can sometimes be moved across namespaces as drafts.
This also causes problems with attribution under our license if the new “creator” didn’t write the entirety of the content.
To replicate this bug:
- Open the source editor on a page
- Have another user move a page to a different title without leaving a redirect.
- Hit publish changes.