Reply or Forward of a closed (or tracked) CRM email could lead to duplicate email activities if the following is true:

CRM Server time is not in sync with client times, e.g. server time is a couple of minutes ahead of the client times.

In this case the browser cache will immediately expire on the client side causing a secondary server request which can result in duplicate activities; and in rare cases also can have effects on plugins configured on the server side to react twice.

Best Regards

Juliane Bock