With some site setups there may be multiple user accounts with the same email address in the users table (this is technically possible with the default schema). However, when that happens, the accounts with matching email addresses can't be updated through the standard UI workflow.
It seems that if someone already has an email address set, that they shouldn't get an error updating other fields on their account. This patch adds an additional check to the form validation so that if their existing email address matches the address on the form, the validation is passed.