Leads with the exact same email listed twice in Marketo

Alex, this can happen a few different ways. Marketo will dedupe leads based on email address based on upload, but it will still accept a lead create call from an external system like Salesforce that has an email address that exists in the system.

You can use the Merge function to bring them back into one lead. It's really easy to use.

In addition to what Charlie said above, if you have non marketo forms on your website and data gets dumped into Marketo, it will still accept the lead and not auto dedupe.  

Here are the most common ways how you may add duplicates to your Marketo instance:

1. You import a list that includes the same email address twice
2. You manually enter duplicates in Marketo, under Lead Database
3. You have duplicates in your CRM (e.g. Leads and Contacts with the same email)

Have you eliminated these possibilities?