You can configure JEvents to block the creation of events that overlap with other events or repeats.
You can set this universally to block any overlapping events or on a category specific basis - this way you can stop the creation of more than one event in the same category at the same time.
Club member addons such as the managed locations addon can be used to block the creation of overlapping events at the same venue. The screenshot here tells the creator of this event that there is already a clashing event occuring at the Ruthin Goal Museum.
Show your support
Unlike many Joomla calendars we do not charge to download JEvents - please show your support for this project by becoming a member of the JEvents Club Club members get access to early releases, exclusive member support forums, and Silver and Gold members can use many exciting JEvents addons
Your membership will ensure that JEvents continues to be the best events calendar for Joomla.