Cécile, in SFDC, when we "merge" a Lead with a Contact using the convert function, the corresponding lead records in Marketo are also merged automatically, so you may have some type of permissions / field blocking configuration setting that is causing the updates not be be synched to Marketo from NetSuite. Also, when we merge the leads in Marketo, they are automatically merged in our CMS.
In the example you provided, were all of these records initially created in Marketo? If so, I'm surprised because it shouldn't create a lead record with a duplicate email address unless that duplicate is synched from your CRM (or via an API call).