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.
Would you happen to know of one you can recommend? Marketo referred us to Vertify during implementation and they weren't able to solve this issue in particular.
You could leverage Demand Tools. I know a customer who has used this tool to match records that are potential duplicates and flag this with their sales team to get them to confirm and merge.
Hope this helps
We are in the same situation, where we also use Marketo with NetSuite and get vendor provided lists of names.
If you're working with Vertify (first of all that sucks cause they suck) but we are in the exact same situation.
Vertify is going to connect the data between NS and Marketo and the way we manage dupes, while it is manual, works well for us.
Once you discover a dupe, we merge records in NS but have the Marketo record in question pulled up for reference. Then we just make sure that we merge the right record so that what stays in NetSuite is clean. Oversimplified maybe based on what people are replying above, but that works for us.