When I wrote this topic, it was about an ordinary + group tags.
Since then, I noticed that it’s also possible to have multiple ordinary tags with the same name, even in the same place in the tag hierarchy.
I would expect the tag filter and the autocompletion to show me all the tags.
The user can act accordingly. It can be a valid repetition of tag names, or it can be a redundancy that should be resolved.
And as I mentioned above, if there is indeed a redundancy, the fact that DEVONthink hides tags makes it more likely to go unnoticed and remain unresolved.