Update - I went back and updated my email address for all UO accounts under a single master to the same email address that is tied to the master, and not a single house entry gave an error about a duplicate address (though one email address is now entered on each house via multipe accounts). Each just said, "This account is registered with the email address [master address here]."
So either that error message is not working properly, or the rules regarding duplicate email addresses have not been properly explained by the dev team.
So either that error message is not working properly, or the rules regarding duplicate email addresses have not been properly explained by the dev team.