If you try to tag a component which is already available in the project you will get a message:
If you want to know for which components you need an unique tag, you can open the processpower.dcf of your project with a SQLite Browser like SQL Expert Personal or SQLite Database Browser.
But be aware that modifying direclty dcf-files is not supported. Make always a backup before.
Here you will find a table called "PnPTagEnlistedColumn". There are all fields listed which have to be unique. So it is by default the tag-field of the tables:
You can modify this list if you want but be aware that, if you delete for example the pipeline group tag from this table, the security and quality aspect of P&ID will be bypassed by you.