We've yet to introduce options for deleting or combining shows via the user interface because these are operations with the potential for a lot of baggage (i.e. people already indicated they attended the show, Phantasy games with user picks are already tied to the show, there are setlists in both shows and they differ, etc.). However, when it comes to back filling a band's history, a lot of this baggage probably isn't there or isn't relevant. In those cases, the best option for a duplicate show is to edit one of the two entries and use it for another show that you've not yet entered. If there simply are no additional shows that need entered at this time, you can disassociate the show from any tours that you've assigned it to and set the date to something in the past (i.e. 1/1/1980). We periodically come through and clean-up shows that are so obviously isolated.
If the two entries have already amassed attendees, setlists, game picks, etc., please submit a support desk ticket to resolve the issue.