The easiest way, as you mentioned, is for accounts to be matched up by email address. When that’s not possible, there are a couple of approaches we use:
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Match up users using some other unique identifier. You can pre-create DiscourseConnect associations for a user via the console. This will only work if you have some other identifier which is consistent between Discourse and the Identity Provider
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(most common) allow users to sign up for new accounts, then get them to message an admin and ask for the new account to be “merged” into the old one (using the button at the bottom of user admin page). This will automatically match up the emails, and create the DiscourseConnect association.