Many thanks to Jep Castelein who pulled much of this information together and tested it.
The best practice within Marketo is to work toward having no duplicate leads , but there are times this happens, especially since Marketo treats SalesForce and Microsoft Dynamics as the source of truth. Duplicates in those systems will result in duplicates in Marketo, and to be clear, a duplicate in Marketo is defined as a lead with the same email address.
I'll use the following as an example
When considering the implications and the behavior of the system with regard to duplicates there are few key areas ..
Batch Campaigns
A batch campaign for which both leads above qualify will only go out to one lead. In this case, it will go out to the one with the LOWER Marketo ID, that is, the one that was created first in Marketo. When the email is sent and clicked on, the activities will be attributed to that lead as well.. and if the link in question leads to a page with munchkin code on it, any activity will also be attributed to this lead.
Trigger Campaigns
These are generally not a problem because they are a "Batch of 1" although they do not de-duplicate in the same way batch campaigns do.
Form Fills
If a lead that exists in Marketo more than once fills out a form, if a cookie exists on the machine they're using, the form filled will be attributed to whatever lead is associated with the cookie.. if NOT, it will be attributed to the lead that was updated most recently in marketo, and of course then the lead would be cookied.
Web tracking
The cookie ID stored within a browser can be linked to only one lead, but of course every browser can have its own cookie, and those can point to different duplicates. This happens when
- A lead clicks a link in an email
- A lead submits a form
List Uploads
Uploads will update the most recently updated duplicate
Lead Scoring
This is a big one. Lead scoring is triggered off of things like form fills, web visits, etc, so it follows the rules above. Obviously your risk here is that you can have a split lead score, with different actions attributed to different leads, leading to artificially low lead scores. Its not a huge problem though since much of the lead activity ends up being associated with the most recently updated lead.
Interesting Moments, Activity Types
Activities can end up also spread over multiple duplicates, as with lead scoring.
Reporting
Another effect of the activity being spread out so you may see lower click and open rates than you might expect. As with the above, its most likely that it's the most recently updated duplicate that is being reported on
So... if your business case requires you to maintain duplicates, just keep this behavior in mind
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