Firstly you need to realise that the calls are coming through on different lines, thus they cannot be duplicate calls.
On an Ericsson (BP250 and MD110) you can make/ receive multiple calls from the same extension at the same time. (An announced transfer also counts as another call). Each of these calls take place on a different line number. And from what we can gather, only once you finish the last call and put down the handset, does the pabx send through the call data. That is why the duration and line numbers differ, but the date & time and the extension are exactly the same.