There is no way to ‘delete’ a tenancy once they have been added. The only way to get "rid" of them is by archiving them so they do not show on your Current Tenancies List.
To do this you will need to set the vacate date for the same day as the start date (must be a date in the past),
(NOTE: To avoid the original tenant from having rent arrears, set the vacate date to a day before the original start date)
Go to Option and write in the comments: Archived due to duplication
Then click save and close and then archive the tenant.