If email address is the unique ID used by MArketo to detect duplicates, how is it possible for leads to get created when an existing lead with the same email address already exists in Marketo?
What is your source of these leads? It's standard functionality for Marketo to honor records from your CRM if they have unique IDs. But Marketo list uploads and form submissions find a matching record by email address before creating a new record.
A couple of scenarios I've seen:
1. Do you have a 1:1 sync with your CRM? Sometimes the sync user between the two platforms is not able to see all leads and contacts in the CRM (called often a "partial sync". If contact/lead that is in your CRM is not visible to the sync user - in effect it is not available for Marketo to see therefore it will attempt to create a new lead. Check your Sync user's permissions to make sure it can see all leads/contacts if you want them all in Marketo.
2. What is the source of your duplicate lead? Was it created in Marketo using a form fill or a list upload? These sources should dedupe but if someone manual created the lead in marketo then you could potentially create a duplicate. If you look at the duplicate record's activity in Marketo you should see the creation source on there to see where it was created.
3. Was the lead/contact a duplicate created in your CRM? Marketo is designed to treat the CRM as a "source of truth" so if the person was created in CRM as duplicate, Marketo will bring it down as a separate record.
Hope this helps!
If email address is the unique ID
Let me just adjust your thinking there: Email Address is most certainly not a unique identifier in Marketo. It's not a primary key, and it's not a unique key.
It is the pre-insert lookup value used by default for some methods of getting leads into Marketo, but not all. Creating a lead via the UI, changing the value in a flow, API-based inserts, CRM syncs -- none of those methods must pass through the same pre-insert lookup.
So really it's only in a controlled situation, where leads are created by Marketo form fills and UI-based CSV uploads only, and the Email Address field is never changed on the server side, where you can expect Email Address to be unique.