I thought I had found a form embed problem, but I think I found something much bigger.
Let's say I fill out a form today using my work email address. A record is created in Marketo, and that record is eventually put into an engagement program.
Let's say I come back in a few days and fill out another form. This time I accidentally put in my home email address.
Several days pass. I get an Engagement Program email. I click on the email and I'm taken to a page with a form on it. It's all pre-filled so I don't even pay attention to the email address.
Here's what's happening in Marketo:
So how do I fix this? I think I need to change the _mkto_tkn on the page to match the value in the URL parameter and to do that I need to decrypt the parameter value and I don't think I can do it before the form is pre-populated with the information on the second record. It also looks like if I change the email address on the form it could potentially create a new record instead of changing the email address on the record itself.
There's no decrypting the mkt_tok. But that's okay, because mkt_tok and the _mkto_trk, while intimately connected, do not share internal information (when you think about how they're generated, it's not possible for one to be a subset/version of the other).
The situation you're describing is probably pretty rare, but is actually an ideal moment to dedupe the two leads, rather than either [a] assigning all the past and future activities to the lead that converted or [b] leaving two segregated activity logs. Some very dirty ways to approach this come to mind already, but let me think more about it before making a recommendation.
OK, but let's say I send you an email to your work address that leads to a page with a form with a social fill option. You click the social button and, because your LinkedIn is tied to your personal email, a new lead is created. Here's your use case.
This is perfect example on why we don't use social sign-on with our forms (since most social account email addresses are personal and not the same as the business email address in Marketo). I actually think this issue is more common than one might think. Especially in top-of-the-funnel engagement, where some prefer to initially provide alternate email addresses until a relationship has been established. It would be great if Marketo included secondary email address fields so that this complete view of the customer can be achieved.