Tag name Must be Unique but Values don't need to be. For Example, it you name a tag "Application" and give it value: EasyTravel, you can tag multiple entities with that tag, but you cannot create a new tag called "Application" again. You can however add additional rules that include other values such as "NotSoEasyTravel" for a Value of "Application"
Ensuring tag "KV" uniqueness per entity would be a desirable feature. However, from my own experience - managed/offline cluster", there's no enforcement and nothing can stop me to duplicate tags with the same key and same or different values. I just posted a question a minute or so ago specifically about how to differentiate auto-applied vs custom tags precisely to avoid/delete duplicates.