1/ Please move this question to Products where it belongs. The specified item was not found. is for generic marketing questions, not Marketo product oriented ones.
2/ Let's look at your problem.
The first thing I would do is to raise a support ticket.
This might be the consequence of some data processing in SFDC, though. Over the years (7+ years working on Marketo, 50+ projects, with a large majority on SFDC), I have never seen such an issue, or even an similar one.
A key rule about the Marketo / SFDC connector is that it does not deduplicate records that come from the CRM.
There are some things I do not understand :
- Of these, a majority are not 1-1, but 1-many email addresses
This is not SFDC standard feature. in SFDC standard, contacts have only1 email address.
You need to look in details to the activity log of a couple of these persons in Marketo. On of the thing I would also look at is their SFDC ID in Marketo (you can display these in a view in a smart list). If the SFDC ID of 2 Marketo duplicates are different, it will really point the cause to Salesforce because this means that the duplicate persons in Marketo do correspond to different SFDC records. If there are the same, then it is likely to be a Marketo sync issue.