This is a known recent issue.
We believe that Mailchimp changed the behaviour of one of their older API calls that we are using. What this is now doing is when you specify a date for everyone sent, it in fact, gives you everyone sent whose merge variables changed since that date. This is not likely to be very many unless many subscribers are changing their merge fields.
We are looking into putting a fix in place but it may take some time.
What we suggest in the mean time is to NOT check the box "Get updated Opens and Clicks from ..." so that all of those records that were sent are brought over.
Chimpegration Classic will not create duplicate records as it checks for the action campaign and existing opens and clicks.