Remove Double Booking permission from System Administrator profile
When installing a package, Salesforce automatically assigns all custom permissions in that package to the System Administrator profile. This is fine for all permissions except the Double Booking permission, as this can give the impression that the conflict checking is not working. Only assign this permission deliberately to users who should always be able to double book.
Go to Setup > Profiles
Go to the SystemAdministrator profile
Under the Custom Permissions section, remove B25.Allow Double Booking
If you have Booker25 installed on your production org, and you create a partial sandbox, Booker25 will most likely not work properly. This is due to not all configuration records being copied to the sandbox. If you want to maintain your production org's Booker25 configuration, there are several options:
Create a full copy sandbox.
Create a partial sandbox and migrate all your Booker25 config data from production to the new sandbox.
Create a partial sandbox and reset the Booker25 config data