How are you all converting/syncing leads that are already contacts in CRM?

This is a terminology issue that comes up a lot and creates confusion.
1. Everyone is called a "lead" in Marketo, but in SFDC, "leads" equals pre-sales prospects and "contacts" are pre-sales prospects that have now made contact with sales and are either in the sales cycle or have become customers.
2. When your CRM contact is synced into Marketo, it is still a contact in your CRM. It won't change into a leads to go through the MQL convertion to contact again.
3. If a lead in Marketo is created and is not a contact in your CRM it will sync to your CRM as a lead (if you sync things that way) and then you can nurture to MQL and convert to contact.
4. In SFDC some orgs convert everyone to a contact, and if you want to watch the lead to contact conversion in that situation you can't to my knowledge. And so people will mass export those names, delete them from salesforce and reimport as new leads into Marketo. YMMV.

Hi Mary - some comments regarding your input above:

1. Understood that everyone is called a lead in Marketo.  In Dynamics CRM, leads are prospects (and MQLs that we sync over from Marketo to be worked on) and can be disqualified (archived) or qualified/converted into contacts and opportunities;  and contacts are customers or potential customers (or an individual related to an account).  Contacts are also attached to opportunities (leads aren't).
2. Correct.  Contacts that are synced to Marketo are nothing more than a lead in Marketo.  But if that lead achieves MQL status and is synced to CRM as an MQL (lead), it won't work and our workflow breaks.

So what we've done to circumvent this (to allow existing CRM contacts to proceed through our lead lifecycle; and allow MQLs that ultimately convert to opportunities/contacts to re-enter our lead lifecycle in the future - which is the norm for us since we primarily sell to existing accounts), is create a dedicated "Duplicate Contacts" lead partition and workspace in Marketo to purposely create duplicates for either of the two scenarios (based on some custom logic).  While this will require us to occassionally de-dupe these records in CRM, it does allow those leads to run through our lead lifecycle process/workflows.

If I get all marketing nerd on this, I would ask why you don't take the existing contacts/customers and take them to a next level of renewal or upsell or customer level 2 etc.

I think one of the frustrations for marketing analysis is when you muddy the test waters with truly new prospects, who then need to go through a particular process to achieve step 1 (become an original customer) and then when they go from first purchase to renewal or follow-on it's a completely different mindset and process and should be tracked separately and differently from a testing and analysis perspective. Combining the two sort of craters that capability and may destroy your long term ability to enhance each separate path.

Sorry for the science rant, but my people are scientific method folks from way back.