Did you ever come across this error when you try to add new domain in windows based plesk panel.

Error: ProtDir_IIS::update() failed: Add Protected Directory failed: columns siteId, path are not unique (Error code 1)

Reason : The protected directory was not synchronized properly, unnecessary entries in IIS Manager > Sites.

Resolution: Access your via Remote desktop ( RDP ) and run below command in CMD.

“%plesk_cli%\repair.exe” –synchronize-protected-directories-storage

This command will synchronize the directories on the server.  After running this command go to IIS manager and remove unnecessary entries of this domain in which you are getting error. You may also need to remove entries  from Mail enable post office. Hope you will able to add the domain now.