Hi Brian,
Creating leads when a contact exists is a quick and sure way to generating issues 
Marketo has some internal rules that you cannot override when attaching activities to a given lead/contact when a duplicates exists.
- Web activities will be attached to the one that is linked to the cookie. As the contact existed first, the contact got the cookie and is collecting the web navigation info, plus the attached score.
- If no cookie exist when the lead in the DB with the lowest ID (created first) will get the activity, so likely the contact as well. But there are some exceptions, especially on import, as described here:
So what you are seing (the contact getting most of the activity) is the expected behavior.
-Greg
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