Hey Gabe,
Thanks for sharing the insight. The Search for an Existing Account feature is in the Web To Lead configuration, in the same area where you configure lead and contact deduping. I use it because I get a lot of new contacts at existing clients through my webforms.
Thanks for linking your other thread, I was seeing something very similar in my instance, but only when I had the account matching feature enabled. I'm currently running (and have been running) Ringlead Lead and Contact deduping on my webforms with no significant issue.
From what I've observed, here's how the existing account matching works: Ringlead checks against matching account variables, and if a match is found, the lead is immediately converted (by Ringlead) to a contact on the matching account. The problem I've seen is that because Ringlead is the one creating/converting the lead in Salesforce, Marketo seems to miss the cue to match its not-yet-synced lead to the existing contact with matching email address. Prior to the Marketo-SFDC sync occuring, I first see a new lead in Market with all the real lead source data (the form they submitted, their lead source, etc). Almost instantly a contact with the same form data appears on the matched account but with a source of Salesforce.com and no lead source .
Then, once the sync catches up, the contact syncs back to Marketo as a new Marketo lead, and the original Marketo lead syncs to SFDC as a new SFDC lead.
So right now, Ringlead (for this particular functionality) is doing exactly the opposite of what I want. I'm currently working with Ringlead's techs, who've escalated it to Marketo to assist. I'll update once I know more.