I've noticed on a few occasions that Marketo will create a duplicate lead even if the email address is the same and am trying to understand why this would happen. Example log:
You can see in the screenshot that there was an existing lead, then a new form fill out occurred and triggered a "New Lead" activity. Both leads had the same email address and were later merged via a third-party tool, which is why you see the combined log now.
It's worth noting the Fill Out Form activity was not a traditional form fill -- it was done via a server-side post (can't find the documentation anymore, looks like Marketo removed it). The posting used a URL construct like this: http://app-sjp.marketo.com/index.php/leadCapture/save?munchkinId=[MYMUNCHKINID]&formid=[FORMID]&FirstName=...
Is this just due to the API nature of the form fill? Or is there another explanation as to why the activity wouldn't just match to the existing lead, rather than create a new one?