How an “unintrusive” 3rd-party tracking script caused duplicate Marketo form posts
The sheer audacity of 3rd-party form trackers (conversion pixels, enrichment services, attribution libraries) is amazing. “Don’t worry about telling IT,” they’ll say. “It has zero side effects, just drop it onto every landing page.”
Now if their code is correctly detecting and using the Marketo Forms JS API, that’d be reassuring. But in reality, most use a Marketo-ignorant — and for that matter, Marketo-competitor-ignorant — approach. They look for <form> elements in the page and at least try, to varying degrees, to listen for native DOM events (focus, change, submit) without disrupting the final outcome.
This approach can easily fail. Take a certain social media conversion tracker that was causing every Marketo form to submit twice with the same data:
This naturally messes up all kinds of Smart Campaigns, and even creates duplicates[1] for net new leads.
How did they screw up? Here’s a trimmed-down equivalent of their code:
htmlForm.addEventListener("subm
