Yeah, I would second this comment from Josh, it depends how the leads are coming into Marketo. If the leads are coming through a form fillout, list upload etc they should be de-duped by email address. This is the only criteria Marketo uses out of the box. If the email address is different (or does not exist) a dupe may be created. If dupes are being created when the lead is coming in through Marketo forms, you definitely need to log this with Support for investigation.
The problem lies if the leads are coming in through your CRM. If a lead already exists in Marketo, and a duplicate lead is created in your CRM, Marketo will NOT de-dupe that lead as it is synced to Marketo from the CRM, even if the email address is the same. In this scenario it is actually in your CRM that you need to make strategic changes to the way information is entered. There are tools for Dynamics that will enable you to prevent dupes being created, if this is the problem that exists you may need to look into this.