To clarify, while it is possible for Marketo Support turn off de-duplication entirely we very strongly recommend against it and it is not considered best practice. Turning off de-duplication runs the risk of quickly running into your lead database limit (resulting in you either needing to delete records from your database or paying more money to increase your database size once you hit the cap) and flooding your CRM and sales team with numerous duplicates of leads. It will also be much harder to manage your Smart Campaigns; leads that should qualify for campaigns if they had a singular record may not, and leads that should not qualify if they had a singular record may.
If you have a legitimate need of duplicate records, please consider adding an additional field for de-duplication instead such as Last Name (this would be a viable solution to Mathias' problem, for instance). A new record would be created if both an email address AND Last Name did not match any existing record in the database in this example. I wouldn't personally recommend any change to duplication rules without spending a good amount of time reviewing the business cases and technical side effects. A consultant would be a good resource for this.