You want to sync a new field in Salesforce to a field in Marketo without creating duplicate fields.
When you create a new field in Salesforce, Marketo will automatically sync that field and create a matching field on the Marketo side, as long as that field is visible to the Marketo Sync User in Salesforce. If the field is on more than one object, such as Lead/Contact or Lead/Account, Marketo will create a single field on the Marketo side as long as you take the following steps.
Disable the global sync in Marketo. (Admin > Salesforce) This will prevent the new fields on the SFDC objects from coming down on separate sync cycles and creating duplicates in Marketo.
Create the field on your desired objects in SFDC. Make sure they both have the same API name, and that the Marketo Sync user can see the field.
Re-enable the global sync in Marketo, allowing the field in both objects to sync down to Marketo at the same time.