@Carissa C -- I think @Bruce J is also alluding to this -- is it possible that you're confused about what cookies do/don't do with respect to the Marketo back end database?
Even with cookies all disabled, a simple form submit will update a Marketo Lead record, and in turn update your CRM. That's kind of the intent of filling out a form!
If you want to use a form without any risk of clobbering an existing lead, you'd have to ensure that you don't update any "real" Marketo fields but rather placeholder fields like pendingEmail, pendingLastName, etc. We do something like this when a user tries to update their own info, putting it in a quarantine of sorts (but this is done on the SFDC side).