Prevent edit user from duplicating email address

ref-test/app/models/user.py

@@ -189,7 +189,10 @@ class User(UserMixin, db.Model):
         if not password and not email: return False, 'There were no changes requested.'
         if password: self.set_password(password)
         old_email = self.get_email()
-        if email: self.set_email(email)
+        if email:
+            for entry in User.query.all():
+                if entry.get_email() == email and not entry == self: return False, f'The email address {email} is already in use.'
+            self.set_email(email)
         db.session.commit()
         write('system.log', f'Information for user {self.get_username()} has been updated by {current_user.get_username()}.')
         if notify: