A user reports that they have inadvertently duplicated some content. There is an FAQ “Suddenly, the map scrolled down and a mess of duplicate items appears.” But I was not able to reproduce this error in the newer versions (Release 41 from Aug 2017), unless this behavior is explicitly selected, via Advanced > Cluster Drag&Drop enabled.
Here is a video that shows how it may happen and what to do then/ how to do block deletes.
Thanks to the user for reporting!
You can always look up your version by clicking on the question mark “?” in the menu bar and then selecting About.