I would be very cautious about any change trying to get rid of such duplicates, because none of the marketing automation systems are set up to do it, in particular regarding unsubscribe statuses. Marketo is set up to use the Email Address as an identifier to make a create vs. update decision. The system's Unsubscribe (as well as marketing suspended, black-listed, and email invalid) is tracked at the lead level, not the email level. If there were a secondary email field, for example, you'd also have to track which email address they are opting in to. How would you determine if they're giving you consent to process their data in compliance with GDPR? Etc. And Marketo is only set up to send emails to a lead's standard Email Address field, not any other custom field. If there were a separate "Email Address" object with it's own fields of unsubscribed, opt-in, etc., that gets associated with the lead, it would be a different story.
However, in my opinion any attempt to create a secondary email address field, whether or not its possible is going to be a bad idea. If you don't end up severely regretting the decision yourself, whoever inherits your Marketo instance will be cursing your name. Anything you do in this direction will simply be going against the grain of how Marketo was designed. It would probably break frequently, and Marketo Support would be unable to help you.
My recommendation would be to look for a CRM process/plugin that can use logic to match a person with pre-existing records based on name, company, etc., and then route them to the same Sales rep and link to the matched existing record via a lookup field.
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