If a user signs in with one email in the buddyup app via FxA, and registers a different account on the website with a different email address, the user should be able to instruct the site to merge those two accounts into a single account. One user object would be deleted, and the other would be updated with the new data. All FKs that pointed to the deleted user will point to the other user. Data will have to be merged in some cases. This may also apply to other kinds of accounts.