It will be interesting to see if there is any change in the frequency of duplicate posts made by those that have made the changes to their CSS.

UPDATE

This duplicate was done as an experiment, it was posted without clicking the create button, it seems to happen to me when I have multiple nodes open and I am writing or previewing a post, moving to another tab and perhaps voting on that node by pressing the Enter key rather than clicking preview / post somehow also changes the focus on the Preview / Create page and the duplicate is created (I think).