Added to list trigger on import: only want it to run on newly added leads

Hi Community!

Question: How can I have Marketo Smart Campaign run a flow only on leads added to a list for the first time?

I maintain a list in Marketo of "All customers using product X". I maintain this list by either importing a weekly list of these customers, or by using the API. Going forward, I would like to "onboard" new customers of product X with a set of emails sent over a few weeks. I would NOT like to onboard the large list of customers already using product X today.

I thought that using the "Added to list" trigger would *only* trigger when a lead was added to the list for the first time (i.e. if a lead was already in the list, and that lead was included in the import, it should NOT trigger). This is not the case. When I import a file into the list, all of the leads in the import trigger.

I need some way to add just the new customers to a separate campaign. I CANNOT use "Lead Created", because new users of product X come from my customer base (and therefore are already leads in Marketo).

One way I can think of is to have a custom field in Marketo (e.g. IsProductXUser) that the import files/API always sets to 1. Then I can use a combination of "lead created trigger", "data value changed trigger", and "is in list filter".

But it feels like Mkto should have this ability to look for *new* adds to a list organically, and my solution feels hacky.

Thanks!

PS, this behavior makes no sense to me. Why does Data Value Change not fire on a new lead created, but Added to List does fire if the lead is already in that list? And where is there detailed documentation on this stuff? The product docs are numbingly simple.