I think I see why that’s happening. The topic importer only looks at email address, but it uses a common importer method to create a new user when it doesn’t find an existing user with the email address. That method will look for a matching username, which is not what I want… This is a bug so I’ll have a look.
Scratch that. It does work by looking only at email addresses. @omarfilip Was your Discourse A site a migration from other forum software?