Replacing Database in High Availability Group Enviroment
With SQL Server 2012, there is only one way to synchronize a database when it is added to an availability group--backup and restore.  So you will need to drop the database from the secondary and restore the primary's backup (or restore with overwrite) when adding the database to the availability group.

With SQL Server 2016 and later, automatic seeding is available for initial synchronization, so the backup/restore process can be skipped if seeding is enabled on the AG.