I Understand that "Pre-Fill doesn't have anything to do w/the common case in which someone submits the form using a secondary email address.". The scenario is not exactly clear here. The example that I am referring to is a different case.
OK, but let's concentrate on the OP. 
"[T]hrough form fills or something else, a second lead record will be created with their personal email" clearly refers to the same human signing up under more than one address.
The question is when the same human is detected by later inspection, what do you do? You can't generally answer that question via Pre-Fill as the form is long gone.
The only one of the same-human cases in which Pre-Fill can be made to help -- it doesn't help out-of-the-box -- is when the lead was presented with their Pre-Filled, original email address and, thinking they were changing their email address, overwrote the email address with their new address, only to create a new lead without realizing it. But having Pre-Fill on or off in this case doesn't do anything to change the behavior.
Rather, what you want to do (and what I always recommend) is if the system Email Address is non-empty at form render time, then mark that input as read-only and give them a link to "Change Address" that hides the original Email Address (keeping the original value, not emptying it) and shows another field like New Email Address in which the end user makes their intended change. Then in a Smart List -- not in the form -- you change the Email Address to the {{Lead.New Email Address}} value.
Thus you can work some real magic to prevent duplicates on the form side that the end user meant to be an update, but Pre-Fill by itself doesn't do that at all.
Say there is a case where a person (Pre existing in the database) receives a second email with a link to a Marketo Landing page, where the Marketo form is embeded, are you saying that the form will not prefill with the information (Including the email address) that is already there in the data base? or is it not possible to prefill the details of an existing lead, in such a scenario?
Yes, in this case, if the session was previously anonymous, the form will be filled with the information of the lead that was sent the email. You can defeat duplicates in this case, but not merely by enabling Pre-Fill -- see method above.
In case a non-Marketo LP is used, where a Marketo form is used, that when I recommended the above article . But I certainly agree that there could be better ways to develop a better JD that whats mentioned in the article.
When any form (not just a Marketo form) is used on a non-Marketo LP and you want to Pre-Fill from the Marketo db, this is what you do: Form Pre-Fill. External sites. No limits. You're welcome. You don't use the REST API.
Hi Kana Lai,
Given the above used case that you mentioned, I know of a Marketo De-Dupe solution by the name of M-clean. It has one time as well as real time de-dupe options. It was very helpful to several of my clients who were in the similar situation. If you are interested, Shailendra Pratap can probably help arrange a demonstration for you.
Best Always,
Karan Hari
Hi Sanford Whiteman,
I Understand, My above explanation was based on a very specific used case which I had faced with one of my clients previously. Thanks for sharing your insights on this as well. The used case here could be different and The Link that you provided is really helpful. Hope our combined inputs was helpful to Kana Lai
Best Always,
Karan Hari
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