"Registration failed. A trusted host with the same name already exists." error is returned when user attempts to save the CA Single Sign-On Configuration in CA Single Sign-On: Config >> CA Modules.
This usually happens when you disabled and attempt to re-enable CA Single Sign-On (with same settings as before) from CA PAM.
Define a different Trusted Host Name
OR
delete the existing Trusted Host object from CA Single Sign On: Administrative UI >> Infrastructure >> Trusted Hosts, before saving the CA Single Sign-On Configuration