Depending on the way your Salesforce instance is set up, the update to show the Lead and Contact versions of the field were mapped together may not have come over. That said, if you reach out to Support they should be able to map these two fields back together on the Marketo end as well.
If you ask Marketo they can map these two fields together for you.
Here's how I prevent this from happening:
1) Create new field in Salesforce on the contact, do not make visible to the Marketo user!
2) Create new field in Salesforce on the lead, do not make visible to the Marketo user!
3) Map field from lead object to contact object
4) Go back to contact and lead objects - make visible, put on page layouts, etc.
If they're mapped before you add them to Marketo, that's the easiest way to ensure that they won't come across as separate fields (although more work).
Open a support case and they will fix it. Ive found that usually it maps over into one field. Some things that may cause that not work is if they are different field types.