Based on the question Is it possible to look for conflicting schedule in calendar app? risen by Rogelio Tan I've built a small example workflow how to identify overlapping events in sharepoint's calendar list.
Note: workflow works just on simple not recurring calendar events!
so this is how the workflow looks like and works.
this is how output might look like:
events in calendar
find the workflow attached.
tested on NW2013 on-prem
Hope it helps to someone!