Standard fields to stop updating from Salesforce to Marketo

Taking a step back here - what's the use case behind updating the Email Address in Salesforce in the first place?  If the email address is changed in Salesforce, Marketo will update the same person record (matched via SFDC Lead/Contact ID). A duplicate record would only be created if A new Lead/Contact is created in Salesforce with the same email, or Records are inserted into Marketo via API/import using a different dedupe key (if you have a custom dedupe key in Marketo). 

Coming to your 2nd question: Marketo does not enforce uniqueness on email addresses, so you can have multiple person records with the same email, and each record is uniquely identified by Lead ID, not email. At send/update time, Marketo will deduplicate by email, so only one email is sent. You should go through the implications of duplicate records in Marketo. But again, taking a step back, do you really need duplicate records in your Marketo DB, however purposeful, after a certain point and volume, working with duplicate records becomes really difficult. All in all, Marketo allows duplicate emails, but it’s not recommended operationally