What could be causing duplicate nodes to be posted? One of the content editors for our site is encountering a duplication error when she posts calendar events. You can see what's happening on the front end here: https://www.library.drexel.edu/events

I'm pretty much brand new to administering drupal (outside of some minor content editing) and am just taking over the site in the interim after an employee left while we seek to fill the position.
We see this (or somthing similar) quite often. Some users create a new node, add text, hit save. So far so good, but sometimes they see an type error or so and go back with the browser Back button, make the corrections and hit then save again. Usually this creates a new node instead of editing the one they think they edited.