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:
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Equipment
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Hand Valves
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Instrumentation
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PipeLineGroup
- Nozzles
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.